Add a comment to explain the coordinate of GetWindowRectInVirtualScreen()

CroppingWindowCapturer::GetWindowRectInVirtualScreen() expects to return a
rectangle in full desktop coordinate.

Bug: webrtc:7950
Change-Id: Id1b5de4d655cfc3f375434a696c77029c684880b
Reviewed-on: https://chromium-review.googlesource.com/651706
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Zijie He <zijiehe@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#19700}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f75daa8cb95ed811eb09dc8ed99ddc34605b1d93