Fix: Add Support to iOS 12 in MediaCapabilities polyfill (#3573)

In iOS, there is no support for MediaSource, and we should have  'file' as the type of the MediaiDecodingConfig, to check whether the content is supported or not. We should only require MediaSource support in the polyfill when the type of MediaiDecodingConfig is 'media-source'. 
Issue: #3530 .

lib/polyfill/media_capabilities.js

@@ -31,11 +31,6 @@ shaka.polyfill.MediaCapabilities = class {
       shaka.log.debug(
           'MediaCapabilities: Native mediaCapabilities support found.');
       return;
-    } else if (!window.MediaSource) {
-      shaka.log.debug(
-          'MediaSource must be available to install mediaCapabilities ',
-          'polyfill.');
-      return;
     }
 
     if (!navigator.mediaCapabilities) {
@@ -64,6 +59,9 @@ shaka.polyfill.MediaCapabilities = class {
     }
 
     if (mediaDecodingConfig.type == 'media-source') {
+      if (!shaka.util.Platform.supportsMediaSource()) {
+        return res;
+      }
       // Use 'MediaSource.isTypeSupported' to check if the stream is supported.
       if (mediaDecodingConfig['video']) {
         const contentType = mediaDecodingConfig['video'].contentType;