[Region Capture] Allow Region Capture across webview borders on CFMs

On CFMs, a <webview> might be used. Earlier we verified that cropTo()
was only used for the WebContents being captured. But on CFMs, it is
permissible to mutate a capture to any WebContents within the
same tree, even across <webview> borders.

Bug: 1499089
Change-Id: Id203abd90d9c9ed15b4bf3e8fb90552c56ae16b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5001632
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1221534}