ES31: Support queries on primitives generated by geometry shader

This patch implements a new query target GL_PRIMITIVES_GENERATED_EXT
for querying the number of primitives generated by geometry shader.

BUG=angleproject:1941
TEST=dEQP-GLES31.functional.geometry_shading.query.primitives_generated*

Change-Id: Icdc0d2d8be4ff4d589972eada2fa1f917d7bae4c
Reviewed-on: https://chromium-review.googlesource.com/963793
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index e7f1e9a..ea6add8 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -983,6 +983,8 @@
             return context->getExtensions().disjointTimerQuery;
         case GL_COMMANDS_COMPLETED_CHROMIUM:
             return context->getExtensions().syncQuery;
+        case GL_PRIMITIVES_GENERATED_EXT:
+            return context->getExtensions().geometryShader;
         default:
             return false;
     }