Make crash_handler volatile to prevent compiler optimizing it away.

This bug manifests itself on Android x64 (in ExceptionHandlerTest::ExternalDumper), but is possible on any platform.

The compiler is unaware of the code which runs in the signal handler which reads this variable. Without volatile the compiler is free to optimise away writes to this variable which it believes are never read.

BUG=381142, 346626
R=thestig@chromium.org

Review URL: https://breakpad.appspot.com/8664002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1343 4c0a9323-5329-0410-9bdc-e9ce6186880e