Allow resetting DXVAVDA while it's currently flushing.

video_decode_accelerator_unittest does this, and there's no fundamental
reason it shouldn't work.

Review-Url: https://codereview.chromium.org/2366213004
Cr-Commit-Position: refs/heads/master@{#423058}

media/gpu/dxva_video_decode_accelerator_win.cc

@@ -1010,12 +1010,15 @@ void DXVAVideoDecodeAccelerator::Reset() {
   DVLOG(1) << "DXVAVideoDecodeAccelerator::Reset";
 
   State state = GetState();
-  RETURN_AND_NOTIFY_ON_FAILURE((state == kNormal || state == kStopped),
-                               "Reset: invalid state: " << state,
-                               ILLEGAL_STATE, );
+  RETURN_AND_NOTIFY_ON_FAILURE(
+      (state == kNormal || state == kStopped || state == kFlushing),
+      "Reset: invalid state: " << state, ILLEGAL_STATE, );
 
   decoder_thread_.Stop();
 
+  if (state == kFlushing)
+    NotifyFlushDone();
+
   SetState(kResetting);
 
   // If we have pending output frames waiting for display then we drop those