Tests for WindowFinder

This change adds several tests for WindowFinder implmenetations.
One of the tests uses ScreenDrawer to create a foreground window or console
window (on Windows) and ensures WindowFinder can return its window_id().
The other ensures WindowFinder always returns kNullWindowId if the |pos| is out
of the screens.

Bug: webrtc:7950
Change-Id: I0cc8794e201c2fa042ea8e693434f1b0fa05b47a
Reviewed-on: https://chromium-review.googlesource.com/639591
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Zijie He <zijiehe@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#19676}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 77b7a1da2d90c46afba21ba38fac63b991b6e88d
9 files changed