Move validation of chromium/preferences_schema.json to its own gulp task

With the way that the `default_preferences.json` file is now generated at build time, the `gulp lint` task is now noticeably slower than before. This slowdown has been, and still is, somewhat annoying during the deployment of new ESLint rules.

Hence this patch, which moves the `chromium/preferences_schema.json` validation from `gulp lint` and into a new `gulp lint-chromium` task instead. *Obviously* this new task is run as part of the `gulp npm-test` task, and thus through `npm test` on Node.js/Travis, such that it's still being tested as before.

gulpfile.js

@@ -1389,9 +1389,7 @@ gulp.task(
   })
 );
 
-gulp.task(
+gulp.task("lint", function(done) {
-  "lint",
-  gulp.series("default_preferences", function(done) {
   console.log();
   console.log("### Linting JS files");
 
@@ -1409,9 +1407,16 @@ gulp.task(
       done(new Error("ESLint failed."));
       return;
     }
+    console.log("files checked, no errors found");
+    done();
+  });
+});
 
+gulp.task(
+  "lint-chromium",
+  gulp.series("default_preferences", function(done) {
     console.log();
-      console.log("### Checking supplemental files");
+    console.log("### Checking supplemental Chromium files");
 
     if (
       !checkChromePreferencesFile(
@@ -1422,10 +1427,7 @@ gulp.task(
       done(new Error("chromium/preferences_schema is not in sync."));
       return;
     }
-
-      console.log("files checked, no errors found");
     done();
-    });
   })
 );
 
@@ -1838,5 +1840,8 @@ gulp.task("externaltest", function(done) {
 
 gulp.task(
   "npm-test",
-  gulp.series(gulp.parallel("lint", "externaltest"), "unittestcli")
+  gulp.series(
+    gulp.parallel("lint", "externaltest", "unittestcli"),
+    "lint-chromium"
+  )
 );