Add and use SkSemaphore

Add and use SkSemaphore

This allows a faster implementation of our SkTaskGroup thread pool.
It also means we don't need SkCondVar (which, remember, isn't supported on XP.)

Doing some testing with SampleApp, this really cuts down on the overhead from SkTaskGroup, e.g. 30% to 10%.

BUG=skia:

Review URL: https://codereview.chromium.org/1192573003