Avoid unnecessary post task during frame delivery.

media/base/video_decoder.h

Index: media/base/video_decoder.h
diff --git a/media/base/video_decoder.h b/media/base/video_decoder.h
index d6abf03e175355e66e1d67962eec16d8311be8cf..5a474b56e21ebdc8177edc6cb351e98c7790f108 100644
--- a/media/base/video_decoder.h
+++ b/media/base/video_decoder.h
@@ -65,6 +65,10 @@ class MEDIA_EXPORT VideoDecoder {
   //    Upon reinitialization, all internal buffered frames will be dropped.
   // 2) This method should not be called during pending decode or reset.
   // 3) No VideoDecoder calls should be made before |init_cb| is executed.
+  // 4) VideoDecoders should take care to run |output_cb| as soon as the frame
+  // is ready (i.e. w/o thread trampolining) since it can strongly affect frame
+  // delivery times with high-frame-rate material.  See Decode() for additional
+  // notes.
   virtual void Initialize(const VideoDecoderConfig& config,
                           bool low_delay,
                           CdmContext* cdm_context,
@@ -84,9 +88,10 @@ class MEDIA_EXPORT VideoDecoder {
   // called again).
   //
   // After decoding is finished the decoder calls |output_cb| specified in
-  // Initialize() for each decoded frame. In general |output_cb| may be called
-  // before or after |decode_cb|, but software decoders normally call
-  // |output_cb| before calling |decode_cb|, i.e. while Decode() is pending.
+  // Initialize() for each decoded frame. |output_cb| is always called before
+  // |decode_cb|. However, |output_cb| may be called before Decode() returns, if
+  // other behavior is desired callers should ensure that |output_cb| will
+  // trampoline as necessary.

[xhwang, 2016/04/25 22:40:28]

Note that we requires that the |decode_cb| must NOT be called from within this
method. If the caller trampolines/posts the output_cb and still want to make
sure output_cb is called before decode_cb, then it needs to post the decode_cb
again. This makes me a bit sad. Maybe we should just say something like "the
calls must handles this case correctly".

[DaleCurtis, 2016/04/25 22:52:16]

Do we need to say anything? If |output_cb| runs while Decode() is in flight, it
will execute before |decode_cb| is posted. I don't think anything else is
necessary but a BindToCurrentLoop.

[DaleCurtis, 2016/04/25 23:07:43]

Discussed offline, while the existing behavior is preserved, the use of "may" in
the comment would cause issues when blindly using BTCL -- so that comment has
been removed in favor of the simpler: "callers must handle this case." Which is
already the case everywhere.

   //
   // If |buffer| is an EOS buffer then the decoder must be flushed, i.e.
   // |output_cb| must be called for each frame pending in the queue and