cros-asan: Fix another crash in chromeos asan bots.

This time, add __asan_unpoison_memory_region in the asan_stub.

BUG=none
TBR=yunlian,sky

Review URL: https://codereview.chromium.org/141273013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245794 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245794