This patch intends to remove the support of GL_OES_geometry_shader
in ANGLE compiler.

To make the implementation simpler, currently we decide to only
support "GL_EXT_geometry_shader" as the extension string of geometry
shader in ANGLE.

This patch also updates all the related shaders in angle_unittests
into R"()" format.

BUG=angleproject:1941
TEST=angle_unittests

Change-Id: Ife9858abeedfb46b02c5c2fb1cda16fa27198511
Reviewed-on: https://chromium-review.googlesource.com/773451
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>