Bug 1185972: P4. Initiate seek as early as conditions permit. r=jwwang

We can initiate a seek as soon as we have a start time and metadata have been read.
However, only enable this feature for MSE as some tests expect events in a specific order.

dom/media/MediaDecoderStateMachine.cpp

@@ -1471,7 +1471,8 @@ MediaDecoderStateMachine::Seek(SeekTarget aTarget)
   NS_ASSERTION(mState > DECODER_STATE_DECODING_METADATA,
                "We should have got duration already");
 
-  if (mState < DECODER_STATE_DECODING || IsDecodingFirstFrame()) {
+  if (mState < DECODER_STATE_DECODING ||
+      (IsDecodingFirstFrame() && !mReader->ForceZeroStartTime())) {
     DECODER_LOG("Seek() Not Enough Data to continue at this stage, queuing seek");
     mQueuedSeek.RejectIfExists(__func__);
     mQueuedSeek.mTarget = aTarget;
@@ -2126,6 +2127,12 @@ MediaDecoderStateMachine::SeekCompleted()
   }
   mPlayDuration = newCurrentTime;
 
+  if (mDecodingFirstFrame) {
+    // We were resuming from dormant, or initiated a seek early.
+    // We can fire loadeddata now.
+    FinishDecodeFirstFrame();
+  }
+
   // Change state to DECODING or COMPLETED now. SeekingStopped will
   // call MediaDecoderStateMachine::Seek to reset our state to SEEKING
   // if we need to seek again.