Do not start sampler thread when CpuProfiler is active

Now that CpuProfiler sends does sampling on the profile event processing thread there is no need to launch sampler thread. The latter is used only for --prof profiler.

BUG=v8:2814
R=bmeurer@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23011029

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16430 ce2b1a6d-e550-0410-aec6-3dcde31c8c00