Bug 1182738. Part 1 - have DecodedStream::GetPosition() return a position including the start time.

dom/media/DecodedStream.cpp

@@ -641,7 +641,10 @@ int64_t
 DecodedStream::GetPosition() const
 {
   ReentrantMonitorAutoEnter mon(GetReentrantMonitor());
-  return mData->GetPosition();
+  // This is only called after MDSM starts playback. So mStartTime is
+  // guaranteed to be something.
+  MOZ_ASSERT(mStartTime.isSome());
+  return mStartTime.ref() + mData->GetPosition();
 }
 
 bool

dom/media/MediaDecoderStateMachine.cpp

@@ -2608,7 +2608,7 @@ int64_t MediaDecoderStateMachine::GetStreamClock() const
 {
   MOZ_ASSERT(OnTaskQueue());
   AssertCurrentThreadInMonitor();
-  return mStreamStartTime + mDecodedStream->GetPosition();
+  return mDecodedStream->GetPosition();
 }
 
 int64_t MediaDecoderStateMachine::GetVideoStreamPosition(TimeStamp aTimeStamp) const