Simplify the thunk targets since we don't support apps as components

mojo/public/c/gles2/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 config("gles2_config") {
   defines = [ "GLES2_USE_MOJO" ]
 }
 
-source_set("gles2") {
+group("gles2") {
+  public_configs = [ "//third_party/khronos:khronos_headers" ]
+  public_deps = [ ":headers" ]
+  deps = [ "//mojo/public/platform/native:gles2_thunks" ]
+}
+
+source_set("headers") {
   sources = [
     "gles2.h",
     "gles2_export.h",
   ]
 
   public_configs = [ ":gles2_config" ]
 
   public_deps = [
     "//mojo/public/c/environment",
     "//mojo/public/c/system",
   ]
 }