This patch intends to support "EXT_geometry_shader" as a valid
extension string in ANGLE GLSL compiler.

We decide to support it because in dEQP-GLES31 all geometry
shader related tests are using "EXT_geometry_shader" instead of
"OES_geometry_shader".

1. Support new extension string "EXT_geometry_shader"
2. Enable geometry shader layout qualifiers with EXT_geometry_shader
3. Enable geometry shader builtins with EXT_geometry_shader

BUG=angleproject:1941
TEST=angle_unittests

Change-Id: Iaedd01a9100ccf56243c957db36ff0c983d17060
Reviewed-on: https://chromium-review.googlesource.com/737933Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>