The patch adds handling of the 'shared' qualifier in the shader
compiler.

BUG=angleproject:1442

TEST=angle_unittests

Change-Id: Iaa288026af0faf2a30e40495faa6ea1f5ff02323
Reviewed-on: https://chromium-review.googlesource.com/413200
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>