Add metadata to media::VideoFrame and plumb it through IPC/MediaStream.

content/browser/media/capture/content_video_capture_device_core.cc

Index: content/browser/media/capture/content_video_capture_device_core.cc
diff --git a/content/browser/media/capture/content_video_capture_device_core.cc b/content/browser/media/capture/content_video_capture_device_core.cc
index e2a51c4594be3d97bc1e36bde2868998cd5f1742..dcaa3535a023051f4d770dbe5ebd6bfeaaad3a7d 100644
--- a/content/browser/media/capture/content_video_capture_device_core.cc
+++ b/content/browser/media/capture/content_video_capture_device_core.cc
@@ -70,6 +70,9 @@ bool ThreadSafeCaptureOracle::ObserveEventAndDecideCapture(
     base::TimeTicks event_time,
     scoped_refptr<media::VideoFrame>* storage,
     CaptureFrameCallback* callback) {
+  // Grab the current time before waiting to acquire the |lock_|.
+  const base::TimeTicks capture_begin_time = base::TimeTicks::Now();
+
   base::AutoLock guard(lock_);
 
   if (!client_)
@@ -141,7 +144,8 @@ bool ThreadSafeCaptureOracle::ObserveEventAndDecideCapture(
   *callback = base::Bind(&ThreadSafeCaptureOracle::DidCaptureFrame,
                          this,
                          frame_number,
-                         output_buffer);
+                         output_buffer,
+                         capture_begin_time);
   return true;
 }
 
@@ -188,6 +192,7 @@ void ThreadSafeCaptureOracle::ReportError(const std::string& reason) {
 void ThreadSafeCaptureOracle::DidCaptureFrame(
     int frame_number,
     const scoped_refptr<media::VideoCaptureDevice::Client::Buffer>& buffer,
+    base::TimeTicks capture_begin_time,
     const scoped_refptr<media::VideoFrame>& frame,
     base::TimeTicks timestamp,
     bool success) {
@@ -201,13 +206,22 @@ void ThreadSafeCaptureOracle::DidCaptureFrame(
 
   if (success) {
     if (oracle_->CompleteCapture(frame_number, &timestamp)) {
-      media::VideoCaptureFormat format = params_.requested_format;
-      // TODO(miu): Passing VideoCaptureFormat here introduces ambiguities.  The
-      // following is a hack where frame_size takes on a different meaning than
-      // everywhere else (i.e., coded size, not visible size).  Will fix in
-      // soon-upcoming code change.
-      format.frame_size = frame->coded_size();
-      client_->OnIncomingCapturedVideoFrame(buffer, format, frame, timestamp);
+      // TODO(miu): Use the locked-in frame rate from AnimatedContentSampler.
+      frame->metadata().SetDouble(media::VideoFrame::kFrameRateMetadataKey,
+                                  params_.requested_format.frame_rate);
+      // Provide capture begin/end timestamps in the VideoFrame metadata.
+      // Casting the int64 internal value of the start and end TimeTickses to
+      // double is reasonable here.  This will guarantee 1us resolution for
+      // values up to 2^53 (e.g., ~285 years since kernel boot).  Alternative
+      // approaches are much more heavyweight (e.g., storing the low and high 32
+      // bits as separate fields, storing as formatted ASCII strings, etc.).
+      frame->metadata().SetDouble(
+          "captureBeginTimeTicks",

[Alpha Left Google, 2015/02/26 21:17:23]

Can these names be defined in video_frame.h as well?

[miu, 2015/02/26 23:25:18]

I'm uncomfortable with starting the practice of putting all the possible
metadata keys in video_frame.h.  The captureBegin/End keys are very specific to
one producer (tab/desktop capture) combined with one consumer (Cast streaming). 
The other issue is where we might centrally locate the definition of the keys,
but there is really no good place (i.e., the tab capture code cannot DEPS on
media/cast and vice-versa).

+          static_cast<double>(capture_begin_time.ToInternalValue()));
+      frame->metadata().SetDouble(
+          "captureEndTimeTicks",
+          static_cast<double>(base::TimeTicks::Now().ToInternalValue()));
+      client_->OnIncomingCapturedVideoFrame(buffer, frame, timestamp);
     }
   }
 }