Include menus & dialogs in frames captured by WindowCapturerWin

This change adds logic to WindowCapturerWin to capture overlapping
owned/pop-up windows (e.g. menus, dialogs, tooltips). This makes window
capture behavior more consistent regardless of whether
CroppingWindowCapturerWin is used & its conditions for using crop-from-
screen capture are met (in ShouldUseScreenCapturer). (I.e. regardless
of OS version, window shape / translucency, occlusion by another
potentially top-most window, or whether the capturing app has opted in
to using the cropping capturer).

Owned/pop-up windows associated with the selected window are enumerated
then captured individually, with their contents composited into the
final frame.

This change also:
- Crops out the top window border (which exposed a bit of the background
  when using the cropping capturer, and resulted in an inconsistent
  appearance compared to the side & bottom borders being cropped out).

Bug: chromium:980864
Change-Id: I81c504848a0c0e6bf122aeff437b400e44944718
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/148302
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#28922}