Introduce a FLUSHED state in media::RendererImpl

The renderer will be in the FLUSHED state after initialization and
after Flush completion, indicating that it's ready to start playback
after StartPlayingFrom call.

Review-Url: https://codereview.chromium.org/2804493002
Cr-Commit-Position: refs/heads/master@{#462592}

diff --git a/media/renderers/renderer_impl.h b/media/renderers/renderer_impl.h
index f55a763..0a30165 100644
--- a/media/renderers/renderer_impl.h
+++ b/media/renderers/renderer_impl.h

@@ -82,8 +82,9 @@
     STATE_UNINITIALIZED,
     STATE_INIT_PENDING_CDM,  // Initialization is waiting for the CDM to be set.
     STATE_INITIALIZING,      // Initializing audio/video renderers.
-    STATE_FLUSHING,
-    STATE_PLAYING,
+    STATE_FLUSHING,          // Flushing is in progress.
+    STATE_FLUSHED,           // After initialization or after flush completed.
+    STATE_PLAYING,           // After StartPlayingFrom has been called.
     STATE_ERROR
   };