Clear reference time after SetMediaTime in video-only time source.

media/base/time_source.h

Index: media/base/time_source.h
diff --git a/media/base/time_source.h b/media/base/time_source.h
index 496eced81a4996f63a1162716df651ec27d6a8fc..9d478d17340fb6e1219a6c0a12ccc466a5f3eeda 100644
--- a/media/base/time_source.h
+++ b/media/base/time_source.h
@@ -54,7 +54,8 @@ class MEDIA_EXPORT TimeSource {
   // time is stopped, timestamps will be converted.
   //
   // Passing an empty |media_timestamps| vector will return the last known media
-  // time as a wall clock time.
+  // time as a wall clock time. After SetMediaTime() and prior to StartTicking()
+  // the returned wall clock time must be zero.
   //
   // Within a single call to GetWallClockTimes() the returned wall clock times
   // are a strictly increasing function of the given media times. There is no