Add CroppedDesktopFrameTest

CroppedDesktopFrame was broken by recent changes unexpectedly, it should be
covered by test cases to avoid this kind of accident.

Bug: chromium:761703
Change-Id: Ib95e4fe19a8bb9799db24fefd3ad154ddd15e53e
Reviewed-on: https://chromium-review.googlesource.com/651448
Commit-Queue: Zijie He <zijiehe@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#19725}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 05ec178808fc8b0da4511cfaac1f734304fd48eb