On Mac OSX system, if retina screen is used, the GetWindowBounds() returns
pre-scaled values instead of system coordinates. So this fix considers
per-monitor scale-factor, and stretchs the DesktopRect.
Bug: chromium:778049
Change-Id: I9dc51e08235eba9b3ef6378eaa15737aa444b0c8
Reviewed-on: https://webrtc-review.googlesource.com/17600
Commit-Queue: Zijie He <zijiehe@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20578}
