[CSC] Complete the implementation of SendWheel()

The application provides (x, y) coordinates of where it wants to deliver
the scroll event. These browser interprets those as referring to the
coordinate space defined by the last video frame the app could observe.

However, the render process associated with the capturer is unaware of
the coordinate space of the captured tab. Therefore, Blink translates
the absolute coordinates which the app supplied into relative
coordinates, where 0 is the leftmost/topmost pixel, and increasing
values up to (but not including) 1 refer to progressively further
pixels towards the right/bottom.

When these relative coordinates are received in the browser process,
they are translated into the actual coordinates over the captured
surface based on its size, and the synthetic scroll event is produced in
the desired location.

Note that in the unlikely event that the resolution or aspect ratio
changes concurrently with the application's call to SendWheel(), the
scroll event will be delivered at a different location than the app
might have intended. The same problem is experienced when users try to
interact with a page directly while its layout changes. It therefore
does not require special handling - we let the issue be.

Bug: 1466247
Change-Id: I9d28f02eb40bca2d198f3629dbe94cf26fea04db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5190733
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Auto-Submit: Elad Alon <eladalon@chromium.org>
Reviewed-by: Tove Petersson <tovep@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1248279}