Bug 1314554 - Don't enter dormant when the media is not seekable. r=jya

This change deals with chained ogg files. A decoding error is encountered
even without seeking when exiting dormant. For now, we disable dormant
for there files that are not seekable to avoid playback looks broken
(due to decode error when exiting dormant) on Firefox.

MozReview-Commit-ID: 72axKPHwoIi

--HG--
extra : rebase_source : f6b27da3d7c0428401d4c0a72f716be5739c5055
extra : source : ea0b9c2dd90206ed4fa41f4eb7446dd9b740e899

dom/media/MediaDecoderStateMachine.cpp

@@ -706,6 +706,12 @@ private:
 
   void StartDormantTimer()
   {
+    if (!mMaster->mMediaSeekable) {
+      // Don't enter dormant if the media is not seekable because we need to
+      // seek when exiting dormant.
+      return;
+    }
+
     auto timeout = MediaPrefs::DormantOnPauseTimeout();
     if (timeout < 0) {
       // Disabled when timeout is negative.