Add another MemorySanitizer annotation to ThreadState::visitStack().

Also, improve the comment.

BUG=chromium:340752
R=ager@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/310823002

git-svn-id: svn://svn.chromium.org/blink/trunk@175363 bbb929c8-8fbe-4397-9dbb-9b2b20218538