Speculative change to improve Mac accessibility performance

This change removes a suspected hot path which causes re-computation of
the role attribute in BrowserAccessibilityCocoa which causes some amount
of tree manager lookup for a presentational frame.

See full analysis for 4/1 in
go/mac-chrome-a11y-perf

R=abigailbklein@google.com

Bug: 1311759
Test: none
Change-Id: Icfb0c61ba63c6ee6e6e6436b5ad485403236624a
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3569288
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#989022}