Use global Context lookup in single-threaded cases. In single-threaded cases we don't need to use a TLS lookup for the current Context. Assume a global context has 3 states: unset, set, and multi-threaded: * global context unset -> set the global context * global context set, matches TLS context -> set the global context * global context set, doesn't match TLS -> set multi-threaded * global context multi-threaded -> leave it This also inlines the global context/TLS check into global_state.h. Increases performance of the binding perf overhead test by up to 25% and the draw call perf test by up to 3%. Bug: angleproject:2824 Change-Id: Ib95953ca918f2408fe72ca83061f4e2d1bd1adad Reviewed-on: https://chromium-review.googlesource.com/1231993 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>