content/renderer/media: Update the include path to i420_video_frame.h

Clients should now include webrtc/video_frame.h which is the right
header for I420VideoFrame type, i420_video_frame.h is just a pass
through header include.

This should help fix pbos' TODO in the WebRTC repo and allow us to land
https://webrtc-codereview.appspot.com/46819004/.

BUG=None
R=tommi@chromium.org

Review URL: https://codereview.chromium.org/1067823002

Cr-Commit-Position: refs/heads/master@{#324334}

content/renderer/media/rtc_video_decoder.cc

@@ -18,8 +18,8 @@
 #include "media/base/bind_to_current_loop.h"
 #include "media/renderers/gpu_video_accelerator_factories.h"
 #include "third_party/skia/include/core/SkBitmap.h"
-#include "third_party/webrtc/common_video/interface/i420_video_frame.h"
 #include "third_party/webrtc/system_wrappers/interface/ref_count.h"
+#include "third_party/webrtc/video_frame.h"
 
 namespace content {