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/generic_decoder.h

@@ -31,6 +31,7 @@ struct VCMFrameInformation {
   void* userData;
   VideoRotation rotation;
   VideoContentType content_type;
+  EncodedImage::Timing timing;
 };
 
 class VCMDecodedFrameCallback : public DecodedImageCallback {
@@ -68,6 +69,7 @@ class VCMDecodedFrameCallback : public DecodedImageCallback {
   rtc::CriticalSection lock_;
   VCMTimestampMap _timestampMap GUARDED_BY(lock_);
   uint64_t _lastReceivedPictureID;
+  int64_t ntp_offset_;
 };
 
 class VCMGenericDecoder {