Make the Linux CrashGenerationClient an interface.

Also allow it to be set on the ExceptionHandler. This will allow Chromium's
implementation to be properly treated as an out-of-process handler.

BUG=https://code.google.com/p/chromium/issues/detail?id=349600
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/2664002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1324 4c0a9323-5329-0410-9bdc-e9ce6186880e