PDF a11y: Set live region only when it's in foreground

Set live region attributes to the status node in PDF a11y tree when it
is in foreground.

tree if its associated PDF tab is in background.

AX-Relnotes: on macOS, VoiceOver won't read the status node in PDF a11y
Bug: 319112722
Change-Id: I23926da23dcd20efa7a4d61ec52613dc4fa2ede9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5304118


Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1261966}