Make the entry point for VideoFrames to webrtc const ref I420VideoFrame.

This removes the none const pointer entry and SwapFrame.

Since frames delivered using VideoSendStream no longer use the external capture module, VideoSendStream will not get an incoming framerate callback. VideoSendStream now uses a rtc::RateTracker.
Also, the video engine must ensure that time stamps are always increasing.

With this, time stamps (ntp, render_time and rtp timestamps ) are checked and set in ViECapturer::OnIncomingCapturedFrame

BUG=1128
R=magjed@webrtc.org, mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46429004

Cr-Commit-Position: refs/heads/master@{#8633}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8633 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/test/vcm_capturer.h

@@ -28,7 +28,7 @@ class VcmCapturer : public VideoCapturer, public VideoCaptureDataCallback {
   void Stop() override;
 
   void OnIncomingCapturedFrame(const int32_t id,
-                               I420VideoFrame& frame) override;  // NOLINT
+                               const I420VideoFrame& frame) override;  // NOLINT
   void OnCaptureDelayChanged(const int32_t id, const int32_t delay) override;
 
  private: