[oilpan]: Explicitly unpoison the header when marking or checking if marked in ASAN.

Now that we are using atomics to read/write the mark bit we cannot rely on NO_SANITIZE_ADDRESS
since it doesn't propagate to called methods (in this case acquireLoad/releaseStore). We don't
want to add NO_SANITIZE_ADDRESS to acquireLoad/releaseStore so instead we explicitly
unpoison/poison the header.

R=ager@chromium.org, haraken@chromium.org, oilpan-reviews@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/544043003

git-svn-id: svn://svn.chromium.org/blink/trunk@181470 bbb929c8-8fbe-4397-9dbb-9b2b20218538