Pass the SIGPROF signal on to previously registered signal handler.

Regressions: Unexpected crashes (2)
  inspector/profiler/cpu-profiler-profiling-without-inspector.html
  inspector/profiler/cpu-profiler-profiling.html

R=danno@chromium.org

Review URL: https://codereview.chromium.org/11377052

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12892 ce2b1a6d-e550-0410-aec6-3dcde31c8c00