Add support for subsampling of SyzyASAN allocation guards to BlockHeapManager. This is mostly a copy of the code living in asan_heap*.* .... BUG= [email protected] Review URL: https://codereview.appspot.com/130070043 git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2257 15e8cca8-e42c-11de-a347-f34a4f72eb7d