Implement GetWindowList() on X11

This change implements GetWindowList() on X11. WindowCapturerLinux and
GetWindowUnderPoint() can share the logic of this function.

Bug: webrtc:7950
Change-Id: Ida746840d6f51d31e0470e5ae4955b6f5a4cfaf2
Reviewed-on: https://chromium-review.googlesource.com/606560
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Zijie He <zijiehe@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#19314}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: ad501d19881fd06a622980fd1d27f95ba89f3348
7 files changed