Implement timing frames.

Timing information is gathered in EncodedImage,
starting at encoders. Then it's sent using RTP header extension. In the
end, it's gathered at the GenericDecoder. Actual reporting and tests
will be in the next CLs.

BUG=webrtc:7594

Review-Url: https://codereview.webrtc.org/2911193002
Cr-Commit-Position: refs/heads/master@{#18659}

webrtc/modules/video_coding/encoded_frame.h

@@ -85,6 +85,10 @@ class VCMEncodedFrame : protected EncodedImage {
    *  Get video content type
    */
   VideoContentType contentType() const { return content_type_; }
+  /**
+   * Get video timing
+   */
+  EncodedImage::Timing video_timing() const { return timing_; }
   /**
    *   True if this frame is complete, false otherwise
    */