Change how CopyMainAndPopupFromSurface computes drop-down offsets

RenderWidgetHostViewBase::CopyMainAndPopupFromSurface was using
GetBoundsInRootWindow() to get the offset of the window and drop-down.
Unfortunately, despite the name, GetBoundsInRootWindow() actually gets
the bounds of the top-level view, not this view. This caused drop-down
windows to be drawn in the wrong place on screenshots. We fixed this
issue by calling GetViewBounds() instead.

Bug: 450618828
Change-Id: I90c2b0636f0c0eedb2771c7a6e8e3e111153e4b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7031505
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/main@{#1529622}