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

The error reported is:
  ERROR logging_AsanCrash logging_AsanCrash timestamp=... localtime=... Unhandled ImportError: /usr/local/autotest/deps/pyauto_dep/test_src/out/Release/_pyautolib.so: undefined symbol: __asan_unpoison_memory_region
  Traceback (most recent call last):
  File /usr/local/autotest/common_lib/test.py, line 480, in _exec
  _cherry_pick_call(self.initialize, *args, **dargs)
  ...

BUG=none
R=yunlian@chromium.org
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/139303019

git-svn-id: http://src.chromium.org/svn/trunk/src/chrome/test/pyautolib@245787 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
diff --git a/asan_stub.c b/asan_stub.c
index d218451..9bbd9d1 100644
--- a/asan_stub.c
+++ b/asan_stub.c
@@ -111,6 +111,7 @@
 void __asan_after_dynamic_init() { }
 int __asan_option_detect_stack_use_after_return;
 typedef unsigned long long uptr;
+void __asan_unpoison_memory_region(void const volatile *addr, uptr size) { }
 uptr __asan_stack_malloc_1(uptr size, uptr real_stack) {}
 uptr __asan_stack_malloc_2(uptr size, uptr real_stack) {}
 uptr __asan_stack_malloc_3(uptr size, uptr real_stack) {}