Reimplement "Avoid Platform.supportsMediaType when loading."

This reverts commit 6667ea5 and
reimplements it using a fallback to an arbitrary media element when
there isn't one yet.

This also adds a regression test that would have caught the issue
sooner.

Closes shaka-project#2326

Change-Id: I44573ae551db51f788d7c97b9680dfd45adee0a0

lib/player.js

@@ -1082,7 +1082,8 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     if (mimeType) {
       // If we have a MIME type, check if the browser can play it natively.
       // This will cover both single files and native HLS.
-      const canPlayNatively = this.video_.canPlayType(mimeType) != '';
+      const mediaElement = payload.mediaElement || Platform.anyMediaElement();
+      const canPlayNatively = mediaElement.canPlayType(mimeType) != '';
 
       // If we can't play natively, then src= isn't an option.
       if (!canPlayNatively) {

lib/util/platform.js

@@ -44,7 +44,7 @@ shaka.util.Platform = class {
    * @return {boolean}
    */
   static supportsMediaType(mimeType) {
-    const video = shaka.util.Platform.anyMediaElement_();
+    const video = shaka.util.Platform.anyMediaElement();
     return video.canPlayType(mimeType) != '';
   }
 
@@ -157,9 +157,8 @@ shaka.util.Platform = class {
    * Cached elements expire in one second so that they can be GC'd or removed.
    *
    * @return {!HTMLMediaElement}
-   * @private
    */
-  static anyMediaElement_() {
+  static anyMediaElement() {
     const Platform = shaka.util.Platform;
     if (Platform.cachedMediaElement_) {
       return Platform.cachedMediaElement_;

test/player_integration.js

@@ -368,6 +368,23 @@ describe('Player', () => {
       expect(video.playbackRate).toBe(1);
     });
 
+    // Regression test for #2326.
+    //
+    // 1. Construct an instance with a video element.
+    // 2. Don't call or await attach().
+    // 3. Call load() with a MIME type, which triggers a check for native
+    //    playback support.
+    //
+    // Note that a real playback may use a HEAD request to fetch a MIME type,
+    // even if one is not specified in load().
+    it('immediately after construction with MIME type', async () => {
+      const testSchemeMimeType = 'application/x-test-manifest';
+      player = new compiledShaka.Player(video);
+      await player.load('test:sintel_compiled', 0, testSchemeMimeType);
+      video.play();
+      await waitUntilPlayheadReaches(eventManager, video, 1, 10);
+    });
+
     /**
      * Gets the language of the active Variant.
      * @return {string}