Fixes issues when using the display arrangement utility. The region
outside of the screens had random data when re-arranging displays
while capturing. This CL makes sure this region is always black.
Bug: webrtc:9703
Change-Id: I1481dd0f1b4584e75926755f9b8a6e5161cd5904
Reviewed-on: https://webrtc-review.googlesource.com/97184
Commit-Queue: Brave Yao <braveyao@webrtc.org>
Reviewed-by: Brave Yao <braveyao@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24589}