This changes to construct a new ShHandle of compiler for each Shader,
and use it to translate the shader source in a background thread.

Bug: chromium:849576

Change-Id: Ib49952c7292321ee6aa1c5996f8f7927f40d8f04
Reviewed-on: https://chromium-review.googlesource.com/1177195
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>