Add mojom interface for AXMode to increase flexibility
in AXMode information sent from browser to renderer

This commit changes the AXMode parameter from a simple int to a
mojom interface in anticipation of sending more information as part of
the accessibility mode state from the browser to the renderer.

Potential future rework of AXMode would include a section
for experimental flags or multiple bit fields covering
node filter, property filter, and on/offscreen position filters

Change-Id: I722748f7137f6031d1f3aadab070050e97ab7d64
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3950054
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Amanda Lin Dietz <aldietz@google.com>
Reviewed-by: Bo Liu <boliu@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1074830}