WindowUnderPoint have different signatures on different platforms, which should
be abstract as an interface.
So this change adds a WindowFinder interface to replace WindowUnderPoint free
function. Meanwhile, this change also includes the implementation of
WindowFinderX11 for X11.
Bug: webrtc:7950
Change-Id: I897a50d4033e713b339b6b6f48b5dbbe601e8db0
Reviewed-on: https://chromium-review.googlesource.com/611745
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Zijie He <zijiehe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19375}