ES31: Implement glDispatchCompute for OpenGL backend

This patch refers to https://chromium-review.googlesource.com/c/380639/

BUG=angleproject:1955
TESTCASE=angle_end2end_tests

Change-Id: Iafd7a6ba2d71c0b332d9267a1260d9dbd9800c02
Reviewed-on: https://chromium-review.googlesource.com/462089
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/renderer/null/ContextNULL.cpp b/src/libANGLE/renderer/null/ContextNULL.cpp
index 51d3a2a..d6fba6f 100644
--- a/src/libANGLE/renderer/null/ContextNULL.cpp
+++ b/src/libANGLE/renderer/null/ContextNULL.cpp
@@ -363,4 +363,9 @@
     return result;
 }
 
+gl::Error ContextNULL::dispatchCompute(GLuint numGroupsX, GLuint numGroupsY, GLuint numGroupsZ)
+{
+    return gl::NoError();
+}
+
 }  // namespace rx