Avoid setting an alternative stack for signals if there is already one

P=Mike Hommey <mh@glandium.org>
R=ted at http://codereview.appspot.com/5573054/

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@918 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/client/linux/handler/exception_handler.h

@@ -228,7 +228,6 @@ class ExceptionHandler {
   const char* next_minidump_id_c_;
 
   const bool handler_installed_;
-  void* signal_stack;  // the handler stack.
   HandlerCallback crash_handler_;
 
   // The global exception handler stack. This is need becuase there may exist