Fix a bug in AsanCrashHandlerTest.SetOnExceptionCallback
Fix a bug in AsanCrashHandlerTest.SetOnExceptionCallback

The callback isn't cleared between the tests, this was preventing from running the tests in a loop.

BUG=
R=chrisha@chromium.org

Review URL: https://codereview.appspot.com/83490044

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@2107 15e8cca8-e42c-11de-a347-f34a4f72eb7d

wangming28/syzygy,google/syzygy,Eloston/syzygy,supriyantomaftuh/syzygy,pombreda/syzygy,ericmckean/syzygy,supriyantomaftuh/syzygy,sebmarchand/syzygy,ericmckean/syzygy,google/syzygy,pombreda/syzygy,sebmarchand/syzygy,google/syzygy,wangming28/syzygy,ericmckean/syzygy,pombreda/syzygy,supriyantomaftuh/syzygy,pombreda/syzygy,sebmarchand/syzygy,sebmarchand/syzygy,supriyantomaftuh/syzygy,ericmckean/syzygy,google/syzygy,wangming28/syzygy,pombreda/syzygy,wangming28/syzygy,sebmarchand/syzygy,ericmckean/syzygy,Eloston/syzygy