Convert SHOUTY_CASE Mojo union tags to kCamelCase in //ui/accessibility/mojom.

This CL was uploaded by git cl split.

R=kinuko@chromium.org

Bug: 1148486
Change-Id: Icda3ddd6a6147f8d79a44d4817619e8e2489f801
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3599612


Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#995402}