Use boolean instead of Atomic32 for is_profiling_ flag

The field is only accessed on the JS thread so it can be converted into bool.

BUG=None

Review URL: https://codereview.chromium.org/12340052

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13731 ce2b1a6d-e550-0410-aec6-3dcde31c8c00