Fix UBSan failure in accessibility_unittests

RelativeToTreeBoundsInternal treats offscreen as an in/out parameter, so
passing in an uninitialized boolean triggers UBSan errors that we're
reading an invalid bit pattern as a boolean.

Since the value is ignored and optional anyway, just pass nullptr. While
I'm here, tweak the parameter name comments to match the style in
https://abseil.io/tips/94, which has less whitespace.

Bug: 1394755
Change-Id: Ia5d8f01d6a16b4a607acd7a4b72e1e1d96b970a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4974022
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214904}