Reland "Do not allow AXMode globals to get out of sync"

This is a reland of commit 39ee890c09e19ff020e5aee645cccf75b97bd57b
Reason for reland: undetected compile error after rebasing and landing.

Original change's description:
> Do not allow AXMode globals to get out of sync
>
> Currently, a global AXMode state is stored in both AXPlatformNode
> and BrowserAccessibilityStateImpl. This is dangerous, because
> they could get out of sync.
>
> A better, longer term fix would be to have a single global stored in
> ui/accessibility. We can do that in a second phase.
>
> - Discourage direct use of AXMode setters in AXPlatformNode. Only
>   BrowserAccessibilityStateImpl should be using those.
> - Remove ScopedAXMode from AXPlatform node so that browser tests are
>   not tempted to use it.
> - Add a DCHECK() to ensure the two states are in sync.
>
> This CL is a prerequisite for a follow-up CL, "Code cleanup in RenderFrameHostImpl", which had failing tests because of mismatched
> globals.
>
> Bug: None
> Change-Id: Ie90c3c756bca5ff41b411cd433a1628759f2aed4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3823683
> Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
> Commit-Queue: Scott Violet <sky@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1039317}

Bug: None
Change-Id: I6cfb54e87f5dd51ae114707c9c96710bfd21c0ed
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3857785
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039708}