fix: Fix mediaCapabilities polyfill on Chromecast

Chromecast firmware now includes limited support for
mediaCapabilities, but we still need to install the polyfill for now.
The platform will let us overwrite the decodingInfo method, but not
the mediaCapabilities object itself.  This fixes the polyfill to
handle this situation.

Change-Id: Ia770457ad8465ec1caf4420ef757303adbe55ce5

lib/polyfill/media_capabilities.js

@@ -37,7 +37,9 @@ shaka.polyfill.MediaCapabilities = class {
       return;
     }
 
-    navigator.mediaCapabilities = /** @type {!MediaCapabilities} */ ({});
+    if (!navigator.mediaCapabilities) {
+      navigator.mediaCapabilities = /** @type {!MediaCapabilities} */ ({});
+    }
     navigator.mediaCapabilities.decodingInfo =
         shaka.polyfill.MediaCapabilities.decodingInfo_;
   }