feat(MediaCap): Always polyfill MediaCap for Chromecast

Since we got some unexpected decodingInfo results from MediaCapabilities
on Chromecast, we should fall back to MediaSource.isTypeSupported() on
Chromecast.

Issue shaka-project#1391

Change-Id: I4947ec78624e98c7039df64ad691e14ece2588e0

lib/polyfill/media_capabilities.js

@@ -8,6 +8,7 @@ goog.provide('shaka.polyfill.MediaCapabilities');
 
 goog.require('shaka.log');
 goog.require('shaka.polyfill');
+goog.require('shaka.util.Platform');
 
 
 /**
@@ -22,7 +23,10 @@ shaka.polyfill.MediaCapabilities = class {
   static install() {
     shaka.log.debug('MediaCapabilities: install');
 
-    if (navigator.mediaCapabilities) {
+    // Since MediaCapabilities is not fully supported on Chromecast yet, we
+    // should always install polyfill for Chromecast.
+    // TODO: re-evaluate MediaCapabilities in the future versions of Chromecast.
+    if (!shaka.util.Platform.isChromecast() && navigator.mediaCapabilities) {
       shaka.log.debug(
           'MediaCapabilities: Native mediaCapabilities support found.');
       return;