GN: Replace mojo/system.gni with forwarding groups

mojo/public/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" ]
 }
 
 # GYP version: mojo/mojo_public.gypi:mojo_gles2
 static_library("gles2") {
@@ skipping 23 matching lines @@
     "../platform/native/gles2_impl_chromium_sync_point_thunks.h",
     "gles2_interface.h",
   ]
 
   if (is_mac) {
     # TODO(GYP): Make it a run-path dependent library.
     # 'DYLIB_INSTALL_NAME_BASE': '@loader_path',
   }
 }
 
+
+# In an is_component_build build, everything can link against //mojo/gles2
+# because it is built as a shared library. However, in a static build,
+# //mojo/gles2 is linked into an executable (e.g., mojo_shell), and must be
+# injected into other shared libraries (i.e., Mojo Apps) that need the mojo
+# gles2 API.
+#
+# For component targets, add //mojo/public/gles2:for_component to your deps
+# section.
+#
+# For shared_library targets (e.g., a Mojo App), add
+# //mojo/public/gles2:for_shared_library to your deps
+
+group("for_shared_library") {
+  if (is_component_build) {
+    deps = [ "//mojo/gles2" ]
+  } else {
+    deps = [ ":gles2" ]
+  }
+}
+
+group("for_component") {
+  if (is_component_build) {
+    deps = [ "//mojo/gles2" ]
+  }
+}