Refactor the DownloadManager initialization in GENERIC/CHROME builds again (PR 8203 follow-up)

In the first commit in PR 8203, I changed how the `DownloadManager` was included/initialized in `GENERIC`/`CHROME` builds.
The change was prompted by the fact that you cannot have conditional `import`s with ES6 modules, and I wanted to avoid bundling the general `DownloadManager` into the various Firefox specific build targets.

What I completely missed though, is that the new code meant that `download_manager.js` will now be pulling in the *entire* viewer (through `app.js`).
This is a *really* stupid mistake on my part, since it causes the `dist/build/pdf_viewer.js` used with the viewer components to now include basically the entire default viewer.

The simplest solution that I could come up with, is to add a `genericcom.js` file (similar to the `firefoxcom.js`/`chromecom.js` files) which will be responsible for importing/initializing the `DownloadManager`.

web/genericcom.js

@@ -0,0 +1,34 @@
+/* Copyright 2017 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { DefaultExternalServices, PDFViewerApplication } from './app';
+import { DownloadManager } from './download_manager';
+
+if (typeof PDFJSDev !== 'undefined' && !PDFJSDev.test('GENERIC')) {
+  throw new Error('Module "pdfjs-web/genericcom" shall not be used outside ' +
+                  'GENERIC build.');
+}
+
+var GenericCom = {};
+
+var GenericExternalServices = Object.create(DefaultExternalServices);
+GenericExternalServices.createDownloadManager = function () {
+  return new DownloadManager();
+};
+PDFViewerApplication.externalServices = GenericExternalServices;
+
+export {
+  GenericCom,
+};