On Windows8/10, we prefer cropping desired window out from a whole screen
capture due to some reasons. The problem is Win10 has an invisible border
around the window. If we leave the border, it will expose background a bit.
This cl is about to always remove the border of desired window on Win8/10.
This will help a lot to capturing still windows during window sharing.
This cl still can't handle the background exposure issue when you move the
target window around during capturing. More investigation is needed.
BUG=chromium:737278
Review-Url: https://codereview.webrtc.org/2973853002
Cr-Commit-Position: refs/heads/master@{#18921}
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