MediaApp a11y: Implement ViewportUpdated(), and related a11y actions.

This method is called from the MediaApp SWA (also known as Gallery),
when the viewport of a PDF changes.
Meanwhile, assistive software can change the viewport using
a handful of accessibility actions, such as "ScrollIntoView".

This patch implements the Mojo plumbing for "ViewportUpdated" and demonstrates how accessibility
actions will be able to change the viewport.

More information about the security implications are in the design doc found at:
https://goto.corp.google.com/bl-pdf-ocr/

AX-Relnotes: n/a.
Bug: b/303133098
Change-Id: I21eb48bf09efcef4a495f05f5141c85f1665d3d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4980628


Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267482}