Add a new FirefoxCom.requestAsync method, to simplify the code in web/firefoxcom.js

There's a fair number of cases where `FirefoxCom.request`-calls are manually wrapped in a Promise to make it asynchronous. We can reduce the amount of boilerplate code in these cases by introducing a new `FirefoxCom.requestAsync` method instead.

Furthermore, a couple of `FirefoxCom.request`-calls in the `DownloadManager` are also changed to be asynchronous rather than using callback-functions.
With this patch, we're thus able to replace a lot of *direct* usages of `FirefoxCom.request` with the new `FirefoxCom.requestAsync` method instead.

web/firefoxcom.js

@@ -29,8 +29,8 @@ class FirefoxCom {
   /**
    * Creates an event that the extension is listening for and will
    * synchronously respond to.
-   * NOTE: It is recommended to use request() instead since one day we may not
+   * NOTE: It is recommended to use requestAsync() instead since one day we may
-   * be able to synchronously reply.
+   *       not be able to synchronously reply.
    * @param {string} action - The action to trigger.
    * @param {Object|string} [data] - The data to send.
    * @returns {*} The response.
@@ -52,6 +52,19 @@ class FirefoxCom {
     return response;
   }
 
+  /**
+   * Creates an event that the extension is listening for and will
+   * asynchronously respond to.
+   * @param {string} action - The action to trigger.
+   * @param {Object|string} [data] - The data to send.
+   * @returns {Promise<any>} A promise that is resolved with the response data.
+   */
+  static requestAsync(action, data) {
+    return new Promise(resolve => {
+      this.request(action, data, resolve);
+    });
+  }
+
   /**
    * Creates an event that the extension is listening for and will, optionally,
    * asynchronously respond to.
@@ -97,58 +110,42 @@ class DownloadManager {
     const blobUrl = URL.createObjectURL(
       new Blob([data], { type: contentType })
     );
-    const onResponse = err => {
-      URL.revokeObjectURL(blobUrl);
-    };
 
-    FirefoxCom.request(
+    FirefoxCom.requestAsync("download", {
-      "download",
-      {
       blobUrl,
       originalUrl: blobUrl,
       filename,
       isAttachment: true,
-      },
+    }).then(error => {
-      onResponse
+      URL.revokeObjectURL(blobUrl);
-    );
+    });
   }
 
   download(blob, url, filename, sourceEventType = "download") {
     const blobUrl = URL.createObjectURL(blob);
-    const onResponse = err => {
-      if (err && this.onerror) {
-        this.onerror(err);
-      }
-      URL.revokeObjectURL(blobUrl);
-    };
 
-    FirefoxCom.request(
+    FirefoxCom.requestAsync("download", {
-      "download",
-      {
       blobUrl,
       originalUrl: url,
       filename,
       sourceEventType,
-      },
+    }).then(error => {
-      onResponse
+      if (error && this.onerror) {
-    );
+        this.onerror(error);
+      }
+      URL.revokeObjectURL(blobUrl);
+    });
   }
 }
 
 class FirefoxPreferences extends BasePreferences {
   async _writeToStorage(prefObj) {
-    return new Promise(function (resolve) {
+    return FirefoxCom.requestAsync("setPreferences", prefObj);
-      FirefoxCom.request("setPreferences", prefObj, resolve);
-    });
   }
 
   async _readFromStorage(prefObj) {
-    return new Promise(function (resolve) {
+    const prefStr = await FirefoxCom.requestAsync("getPreferences", prefObj);
-      FirefoxCom.request("getPreferences", prefObj, function (prefStr) {
+    return JSON.parse(prefStr);
-        const readPrefs = JSON.parse(prefStr);
-        resolve(readPrefs);
-      });
-    });
   }
 }
 
@@ -254,13 +251,10 @@ class FirefoxComDataRangeTransport extends PDFDataRangeTransport {
 
 class FirefoxScripting {
   static async createSandbox(data) {
-    return new Promise(resolve => {
+    const success = await FirefoxCom.requestAsync("createSandbox", data);
-      FirefoxCom.request("createSandbox", data, resolve);
-    }).then(success => {
     if (!success) {
       throw new Error("Cannot create sandbox.");
     }
-    });
   }
 
   static async dispatchEventInSandbox(event) {
@@ -343,9 +337,7 @@ class FirefoxExternalServices extends DefaultExternalServices {
   }
 
   static async fallback(data) {
-    return new Promise(resolve => {
+    return FirefoxCom.requestAsync("fallback", data);
-      FirefoxCom.request("fallback", data, resolve);
-    });
   }
 
   static reportTelemetry(data) {