Replace VideoDecoder::media_format() with significantly simpler width()/height() methods.

media/base/filters.h

Index: media/base/filters.h
diff --git a/media/base/filters.h b/media/base/filters.h
index e541c78d6c1a519710a5031777024ba55cc035b5..ffd7201adf8cfbf6bc1b044365fec2952879413b 100644
--- a/media/base/filters.h
+++ b/media/base/filters.h
@@ -225,8 +225,15 @@ class VideoDecoder : public Filter {
   // Indicate whether decoder provides its own output buffers
   virtual bool ProvidesBuffer() = 0;
 
-  // Returns the media format produced by this decoder.
-  virtual const MediaFormat& media_format() = 0;
+  // Returns the width and height of decoded video in pixels.
+  //
+  // Clients should NOT rely on these values to remain constant. Instead, use
+  // the width/height from decoded video frames themselves.
+  //
+  // TODO(scherkus): why not rely on prerolling and decoding a single frame to
+  // get dimensions?
+  virtual int width() = 0;
+  virtual int height() = 0;
 
  protected:
   // Executes the permanent callback to pass off decoded video.