[api-minor] Refactor fetching of built-in CMaps to utilize a factory on the display side instead, to allow users of the API to provide a custom CMap loading factory (e.g. for use with Node.js)

Currently the built-in CMap files are loaded in `src/core/cmap.js` using `XMLHttpRequest` directly. For some environments that might be a problem, hence this patch refactors that to instead use a factory to load built-in CMaps on the main thread and message the data to the worker thread.

This is inspired by other recent work, e.g. the addition of the `CanvasFactory`, and to a large extent on the IRC discussion starting at http://logs.glob.uno/?c=mozilla%23pdfjs&s=12+Oct+2016&e=12+Oct+2016#c53010.

src/core/worker.js

@@ -729,15 +729,10 @@ var WorkerMessageHandler = {
 
       ensureNotTerminated();
 
-      var cMapOptions = {
-        url: data.cMapUrl === undefined ? null : data.cMapUrl,
-        packed: data.cMapPacked === true
-      };
       var evaluatorOptions = {
         forceDataSchema: data.disableCreateObjectURL,
         maxImageSize: data.maxImageSize === undefined ? -1 : data.maxImageSize,
         disableFontFace: data.disableFontFace,
-        cMapOptions: cMapOptions,
         disableNativeImageDecoder: data.disableNativeImageDecoder,
       };