Annotate ThreadState::visitStack() for MemorySanitizer.

This function scans the stack for pointers, and by design may
access uninitialized data. Use a MemorySanitizer annotation
to mark the pointers we pick up as initialized to suppress
MemorySanitizer reports.

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

Review URL: https://codereview.chromium.org/289783002

git-svn-id: svn://svn.chromium.org/blink/trunk@174053 bbb929c8-8fbe-4397-9dbb-9b2b20218538