Decouple capture timestamp and reference time

media/capture/video/win/video_capture_device_win.cc

Index: media/capture/video/win/video_capture_device_win.cc
diff --git a/media/capture/video/win/video_capture_device_win.cc b/media/capture/video/win/video_capture_device_win.cc
index b6018cb4202a1ef8ae657b985cac66ed46bfa2bf..c9194dc87272a21b7d4dbc6e517c0765379bc9e3 100644
--- a/media/capture/video/win/video_capture_device_win.cc
+++ b/media/capture/video/win/video_capture_device_win.cc
@@ -15,6 +15,7 @@
 #include "base/strings/sys_string_conversions.h"
 #include "base/win/scoped_co_mem.h"
 #include "base/win/scoped_variant.h"
+#include "media/base/timestamp_constants.h"
 
 using base::win::ScopedCoMem;
 using base::win::ScopedComPtr;
@@ -451,9 +452,13 @@ void VideoCaptureDeviceWin::StopAndDeAllocate() {
 // Implements SinkFilterObserver::SinkFilterObserver.
 void VideoCaptureDeviceWin::FrameReceived(const uint8_t* buffer,
                                           int length,
-                                          base::TimeTicks timestamp) {
+                                          base::TimeDelta timestamp) {
+  if (first_ref_time_.is_null())
+    first_ref_time_ = base::TimeTicks::Now();
+  if (timestamp == media::kNoTimestamp())

[miu, 2016/05/18 22:35:40]

Please add a comment that sometimes the platform does not provide a timestamp
from the video capture hardware, and so this code is a workaround to derive a
timestamp from the system clock.

BTW--Do we know why/when the platform would not provide a timestamp? If so, it's
probably worth documenting that in a comment here or in sink_input_pin_win.cc.

[qiangchen, 2016/05/20 17:55:14]

In my testing, I did not see a case that timestamp is not provided by OS API,
but that API do have a return value to indicate whether a timestamp is provided
or not. So we need to handle it in case a timestamp is not provided. 

I think the API's implementation could differ from camera to camera, and thus it
is safe we do have a fall back solution here.

+    timestamp = base::TimeTicks::Now() - first_ref_time_;
   client_->OnIncomingCapturedData(buffer, length, capture_format_, 0,
-                                  timestamp);
+                                  base::TimeTicks::Now(), timestamp);
 }
 
 bool VideoCaptureDeviceWin::CreateCapabilityMap() {