WebLayer: clean up MediaSessionHelper when TabImpl is destroyed.

Bug: none
Change-Id: I2f242f1c14dc9ce032f4233ae686c651a7ef9f5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508334Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822799}

weblayer/browser/java/org/chromium/weblayer_private/TabImpl.java

@@ -970,6 +970,11 @@ public final class TabImpl extends ITab.Stub implements LoginPrompt.Observer {
         mMediaStreamManager.destroy();
         mMediaStreamManager = null;
 
+        if (mMediaSessionHelper != null) {
+            mMediaSessionHelper.destroy();
+            mMediaSessionHelper = null;
+        }
+
         // Destroying FaviconCallbackProxy removes from mFaviconCallbackProxies. Copy to avoid
         // problems.
         Set<FaviconCallbackProxy> faviconCallbackProxies = mFaviconCallbackProxies;