Implement IsOccluded() in each DesktopCapturer implementation

This change implements IsOccluded() function in each DesktopCapturer
implementation.

This change was reviewed at
https://chromium-review.googlesource.com/c/external/webrtc/+/660817.

Bug: webrtc:7950
Change-Id: Ie4d49b2bfcf5f0da1576f47b752a51ec494442ad
TBR: jamiewalch@chromium.org
Reviewed-on: https://webrtc-review.googlesource.com/1400
Reviewed-by: Zijie He <zijiehe@google.com>
Commit-Queue: Zijie He <zijiehe@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#19840}
Cr-Mirrored-From: https://webrtc.googlesource.com/src
Cr-Mirrored-Commit: 9cad5016a3e1b2e09dfe05ea24c50fa4327a4384