Add support for the HEAP_ZERO_MEMORY flag to HeapAlloc.

BUG=
R=chrisha@chromium.org

Review URL: https://codereview.appspot.com/139820043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2280 15e8cca8-e42c-11de-a347-f34a4f72eb7d