Backlight OCR: Implements the "hit test" accessibility action.

When a hit test accessibility action is performed, the assistive
software provides a screen point and expects to get back
the accessibility node under that point. This is primarily used by
Select-to-Speak, which is a piece of assistive software
on ChromeOS that reads the text that the user points to using the
mouse.

When a PDF is opened in the Media App on ChromeOS, we perform OCR on
all the PDF content, get bounding rectangles from the OCR engine,
thus allowing the Select-to-Speak user to point to any on-screen
item to have it read.

Security considerations:
The renderer controls the tree ID of the OCRed content that it
provides to the hit test action for that
content.
However:
the stitched child tree ID is an unguessable token and thus hard to
come up with for any random renderer,
the renderer already provides the node ID for the hit test target and
could thus provide any target it wants in that renderer - so why
should there be a need to maskarate as another renderer.
There is no accessibility tree on the browser side for the renderer
content that can therefore be used to validate the renderer provided
tree ID.
The renderer does not control the event that should be fired,
nor the request ID of the hit test action.

Design doc at:
https://docs.google.com/document/d/1k8OH4XGjZrLVA705Xfy6vzSwGL9DkfRt9DG5s66lRjQ/edit?usp=sharing&resourcekey=0-K_9Y1EMhNoo8Ez9RPbQNMA

R=kyungjunlee@google.com

AX-Relnotes: Select-to-Speak now works in the MediaApp, AKA Backlight.
Change-Id: Ic6b0eb764aa2c27c5f145a23be65973f0bca9c40
Bug: 342563905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6000136
Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382208}