media: Add WaitingCB and WaitingReason

- Add WaitingCB that is the generic callback to indicate that the media
  pipeline is stalling/waiting. The status for waiting is indicated in
  WaitingReason.
- Replace WaitingForDecryptionKeyCB with WaitingCB.

Bug: 858286
Test: No functionality change.
Change-Id: Ic709114e3a7ad94a2a9d0682c026815a306807a9
Reviewed-on: https://chromium-review.googlesource.com/c/1374200
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617580}

diff --git a/media/renderers/renderer_impl.h b/media/renderers/renderer_impl.h
index 18f6384..1a4d76f 100644
--- a/media/renderers/renderer_impl.h
+++ b/media/renderers/renderer_impl.h

@@ -25,6 +25,7 @@
 #include "media/base/pipeline_status.h"
 #include "media/base/renderer.h"
 #include "media/base/video_decoder_config.h"
+#include "media/base/waiting.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace base {
@@ -189,7 +190,7 @@
   // Callback executed when a runtime error happens.
   void OnError(PipelineStatus error);
 
-  void OnWaitingForDecryptionKey();
+  void OnWaiting(WaitingReason reason);
   void OnVideoNaturalSizeChange(const gfx::Size& size);
   void OnAudioConfigChange(const AudioDecoderConfig& config);
   void OnVideoConfigChange(const VideoDecoderConfig& config);