Revert of Delete cricket::VideoFrame::GetTimeStamp. (patchset #1 id:1 of https://codereview.webrtc.org/2305623002/ )

Reason for revert:
Broke downstream project.

Original issue's description:
> Delete cricket::VideoFrame::GetTimeStamp.
>
> TBR=tkchin@webrtc.org  # Trivial change to VideoRendererAdapter
> BUG=webrtc:5682
>
> Committed: https://crrev.com/fd6c99e43137d01fa6c120f7160f7c2999d1d8a3
> Cr-Commit-Position: refs/heads/master@{#14037}

TBR=perkj@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5682

Review-Url: https://codereview.webrtc.org/2306953002
Cr-Commit-Position: refs/heads/master@{#14038}

webrtc/sdk/objc/Framework/Classes/RTCVideoRendererAdapter.mm

@@ -30,8 +30,7 @@ class VideoRendererAdapter
     RTCVideoFrame* videoFrame = [[RTCVideoFrame alloc]
         initWithVideoBuffer:nativeVideoFrame.video_frame_buffer()
                    rotation:nativeVideoFrame.rotation()
-                timeStampNs:nativeVideoFrame.timestamp_us() *
-                                rtc::kNumNanosecsPerMicrosec];
+                timeStampNs:nativeVideoFrame.GetTimeStamp()];
     CGSize current_size = (videoFrame.rotation % 180 == 0)
                               ? CGSizeMake(videoFrame.width, videoFrame.height)
                               : CGSizeMake(videoFrame.height, videoFrame.width);