Re-instated deleted code from previous commit, which prevented cast l…

…ab failures.

lib/polyfill/media_capabilities.js

@@ -267,6 +267,10 @@ shaka.polyfill.MediaCapabilities = class {
           shaka.util.Error.Severity.CRITICAL,
           shaka.util.Error.Category.CAST,
           shaka.util.Error.Code.CAST_API_UNAVAILABLE);
+    } else if (!(cast.__platform__ && cast.__platform__.canDisplayType)) {
+      shaka.log.warning('Expected cast APIs to be available! Falling back to ' +
+          'MediaSource.isTypeSupported() for type support.');
+      return MediaSource.isTypeSupported(contentType);
     }
 
     let displayType = contentType;

test/polyfill/media_capabilities_unit.js

@@ -211,6 +211,28 @@ describe('MediaCapabilities', () => {
       expect(isChromecastSpy).toHaveBeenCalledTimes(2);
     });
 
+    it('falls back to isTypeSupported() when canDisplayType() missing',
+        async () => {
+          // We only set the cast namespace, but not the canDisplayType() API.
+          window['cast'] = {};
+          const isChromecastSpy =
+              spyOn(shaka['util']['Platform'],
+                  'isChromecast').and.returnValue(true);
+          const isTypeSupportedSpy =
+              spyOn(window['MediaSource'], 'isTypeSupported')
+                  .and.returnValue(true);
+
+          shaka.polyfill.MediaCapabilities.install();
+          await navigator.mediaCapabilities.decodingInfo(mockDecodingConfig);
+
+          expect(mockCanDisplayType).not.toHaveBeenCalled();
+          // 1 (during install()) + 1 (for video config check).
+          expect(isChromecastSpy).toHaveBeenCalledTimes(2);
+          // 1 (fallback in canCastDisplayType()) +
+          // 1 (mockDecodingConfig.audio).
+          expect(isTypeSupportedSpy).toHaveBeenCalledTimes(2);
+        });
+
     it('should use cast.__platform__.canDisplayType for "supported" field ' +
         'when platform is Cast', async () => {
       // We're using quotes to access window.cast because the compiler