Renamed SignalSender to SamplerThread to match platforms that don't support signal sending.

Cygwin and Windows implementations are almost identical. Mac OS X uses different API to suspend thread but apart from that it is very similar to Windows implenenation.

BUG=None

Review URL: https://codereview.chromium.org/14264009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14272 ce2b1a6d-e550-0410-aec6-3dcde31c8c00