Add another MemorySanitizer annotation to ThreadState::visitStack().
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@175363 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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