Support is added for compute shader compilation. There is a small
extension to the parser so that 'local_size_x = ', 'local_size_y = '
and 'local_size_z = ' are supported as layout qualifiers.

A few shader compilation tests are added and one which checks the AST
whether the layout qualifiers are properly parsed.

BUG=angleproject:1442

TEST=angle_unittests
TEST=angle_end2end_tests

Change-Id: I67283797d1cf13fa4ac47faa2a6e66d93a2db867
Reviewed-on: https://chromium-review.googlesource.com/362300Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>