Nuke SamplerRegistry

The registry is a simple list of active Samplers but uses additional Mutex. Useful parts were merged into SamplerThread, others removed completely.

BUG=None

Review URL: https://codereview.chromium.org/14293009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14297 ce2b1a6d-e550-0410-aec6-3dcde31c8c00