Shared Raster Worker Threads

This CL will share worker pool threads across LTHI for a single process.
Idea is to reduce the number of threads used per process.
For this, we implement WorkerPool::Inner as singleton, and LTHI/WorkerPool
would share the same instance per process.

Reviewer: reveman@chromium.org
BUG=239423

Review URL: https://codereview.chromium.org/73923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245526 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245526