MediaApp/A11y: Create a new mojo pipe for every new PDF document loaded.

Currently, a connection between the Media App and the a11y code running
OCR is established for each window. Switch to making a connection for
every document instead for a couple reasons:
 - Loading a new document means all the information we already have from
   OCR needs to be thrown out anyway, and
 - This avoids having to overwrite / resize the internal AXTrees in the
   A11y service on new document load, because instead everything is
   being destroyed. This provides better security guarantees.

Also see the original suggestion in code review at
https://crrev.com/c/5016238/9.

Change-Id: I6569683e2d7b19c25d8b490696c06c281215909f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5094174
Commit-Queue: Patti Lor <patricialor@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1236059}