Cleanup //ui/content_accelerators from LaCros support code.

No functional changes are done.

1) Renamed IS_CHROMEOS_ASH to IS_CHROMEOS
2) Removed chromeos_buildflags.h includes and deps
3) General clean ups.

Bug: 373972275, 377410076
Change-Id: I722e0e0aed8aa38c79d4d6cf30844fb2b21bc22a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5993581


Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378911}