Fix the NV12 plane size calculation of the UV plane

Bug: 982201
Bug: b:144805331
Change-Id: I8c038c8912aaae48744d318398d85ce6c8fdfa16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1954822Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Ricky Liang <jcliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722423}

third_party/blink/renderer/platform/peerconnection/rtc_video_encoder.cc

@@ -907,7 +907,7 @@ bool RTCVideoEncoder::Impl::CreateBlackGpuMemoryBufferFrame(
   memset(static_cast<uint8_t*>(gmb->memory(0)), 0x0,
          gmb->stride(0) * gmb_size.height());
   memset(static_cast<uint8_t*>(gmb->memory(1)), 0x80,
-         gmb->stride(1) * gmb_size.height());
+         gmb->stride(1) * gmb_size.height() / 2);
   gmb->Unmap();
 
   gpu::MailboxHolder empty_mailboxes[media::VideoFrame::kMaxPlanes];