Reland r164961 with a typo fix for Android.

Use OS APIs to fetch start of the stack for OilPan's conservative stack scanning.

Originally reviewed at https://codereview.chromium.org/131363003/

BUG=

Review URL: https://codereview.chromium.org/136383003

git-svn-id: svn://svn.chromium.org/blink/trunk@164978 bbb929c8-8fbe-4397-9dbb-9b2b20218538