fix: Fix exceptions when quickly shutting down src= on Safari (#4088)

Some events and timers were used to process track changes with src=
playback on Safari, but they did not properly clean up when the player
was unloaded or destroyed.  In the case that this happened quickly
after starting playback, exceptions would be thrown or tracks could
manipulated after new content began.

This fixes the cleanup of these timers and events to be aware of
Player unloads or destruction.

Closes #4087

lib/player.js

@@ -2217,6 +2217,14 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
 
     this.playhead_ = new shaka.media.SrcEqualsPlayhead(has.mediaElement);
 
+    // This flag is used below in the language preference setup and
+    // track-management to check if this load was canceled before the necessary
+    // events fired.
+    let unloaded = false;
+    this.cleanupOnUnload_.push(() => {
+      unloaded = true;
+    });
+
     if (has.startTime != null) {
       this.playhead_.setStartTime(has.startTime);
     }
@@ -2268,6 +2276,12 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     }
     if (this.video_.textTracks) {
       this.eventManager_.listen(this.video_.textTracks, 'addtrack', (e) => {
+        // If we have moved on to another piece of content while waiting for
+        // the above event, we should not process tracks here.
+        if (unloaded) {
+          return;
+        }
+
         const trackEvent = /** @type {!TrackEvent} */(e);
         if (trackEvent.track) {
           const track = trackEvent.track;
@@ -2327,13 +2341,6 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
           fullyLoaded.resolve();
         });
 
-    // This flag is used below in the language preference setup to check if this
-    // load was canceled before the necessary events fire.
-    let unloaded = false;
-    this.cleanupOnUnload_.push(() => {
-      unloaded = true;
-    });
-
     // We can't switch to preferred languages, though, until the data is loaded.
     shaka.util.MediaReadyState.waitForReadyState(this.video_,
         HTMLMediaElement.HAVE_CURRENT_DATA,
@@ -2488,12 +2495,16 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     // In Safari the initial assignment does not always work, so we schedule
     // this process to be repeated several times to ensure that it has been put
     // in the correct mode.
-    new shaka.util.Timer(() => {
+    const timer = new shaka.util.Timer(() => {
       const textTracks = this.getMetadataTracks_();
       for (const textTrack of textTracks) {
         textTrack.mode = 'hidden';
       }
-    }).tickNow().tickAfter(/* seconds= */ 0.5);
+    }).tickNow().tickAfter(0.5);
+
+    this.cleanupOnUnload_.push(() => {
+      timer.stop();
+    });
   }
 
 
@@ -2561,12 +2572,16 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     // In Safari the initial assignment does not always work, so we schedule
     // this process to be repeated several times to ensure that it has been put
     // in the correct mode.
-    new shaka.util.Timer(() => {
+    const timer = new shaka.util.Timer(() => {
       const chaptersTracks = this.getChaptersTracks_();
       for (const chaptersTrack of chaptersTracks) {
         chaptersTrack.mode = 'hidden';
       }
-    }).tickNow().tickAfter(/* seconds= */ 0.5);
+    }).tickNow().tickAfter(0.5);
+
+    this.cleanupOnUnload_.push(() => {
+      timer.stop();
+    });
   }
 
   /**