[api-minor] Refactor/simplify the PDFObject class

First of all, note how there's currently *two* methods for checking if a certain object exists, which seems completely unwarranted.
Furthermore, the rarely used `getData` method was removed and its only callsite changed to use a combination of `PDFObjects.{has, get}` instead.
Finally, the methods were rearranged slightly, to bring the most important ones (for an API user) to the top of the class.

src/display/annotation_layer.js

@@ -447,8 +447,9 @@ class TextWidgetAnnotationElement extends WidgetAnnotationElement {
       element.style.display = 'table-cell';
 
       let font = null;
-      if (this.data.fontRefName) {
-        font = this.page.commonObjs.getData(this.data.fontRefName);
+      if (this.data.fontRefName &&
+          this.page.commonObjs.has(this.data.fontRefName)) {
+        font = this.page.commonObjs.get(this.data.fontRefName);
       }
       this._setTextStyle(element, font);
     }

src/display/api.js

@@ -1885,7 +1885,7 @@ class WorkerTransport {
       }
 
       const [id, type, exportedData] = data;
-      if (this.commonObjs.hasData(id)) {
+      if (this.commonObjs.has(id)) {
         return;
       }
 
@@ -1937,7 +1937,7 @@ class WorkerTransport {
 
       const [id, pageIndex, type, imageData] = data;
       const pageProxy = this.pageCache[pageIndex];
-      if (pageProxy.objs.hasData(id)) {
+      if (pageProxy.objs.has(id)) {
         return;
       }
 
@@ -2256,6 +2256,11 @@ class PDFObjects {
     return obj.data;
   }
 
+  has(objId) {
+    const obj = this._objs[objId];
+    return (obj ? obj.resolved : false);
+  }
+
   /**
    * Resolves the object `objId` with optional `data`.
    */
@@ -2267,26 +2272,6 @@ class PDFObjects {
     obj.capability.resolve(data);
   }
 
-  isResolved(objId) {
-    const obj = this._objs[objId];
-    return (obj ? obj.resolved : false);
-  }
-
-  hasData(objId) {
-    return this.isResolved(objId);
-  }
-
-  /**
-   * Returns the data of `objId` if the object exists, null otherwise.
-   */
-  getData(objId) {
-    const obj = this._objs[objId];
-    if (!obj || !obj.resolved) {
-      return null;
-    }
-    return obj.data;
-  }
-
   clear() {
     this._objs = Object.create(null);
   }

src/display/canvas.js

@@ -810,7 +810,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
 
             // If the promise isn't resolved yet, add the continueCallback
             // to the promise and bail out.
-            if (!objsPool.isResolved(depObjId)) {
+            if (!objsPool.has(depObjId)) {
               objsPool.get(depObjId, continueCallback);
               return i;
             }