GN: Replace mojo/system.gni with forwarding groups

mojo/system.gni

-# 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.
-
-# The following mojo_system-prefixed variables are used to express a dependency
-# on the mojo system APIs.
-#
-# In an is_component_build build, everything can link against
-# mojo_system_impl because it is built as a shared library. However, in a static
-# build, mojo_system_impl 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 system API.
-#
-# For component targets, add mojo_system_for_component to your deps section.
-# For shared_library targets (e.g., a Mojo App), add
-# mojo_system_for_shared_library to your deps
-
-if (is_component_build) {
-  mojo_system_for_component = [ "//mojo/system" ]
-  mojo_system_for_shared_library = [ "//mojo/system" ]
-  mojo_gles2_for_component = [ "//mojo/gles2" ]
-  mojo_gles2_for_shared_library = [ "//mojo/gles2" ]
-} else {
-  mojo_system_for_component = [] # nothing to link against here
-  mojo_system_for_shared_library = [ "//mojo/public/c/system" ]
-  mojo_gles2_for_component = []
-  mojo_gles2_for_shared_library = [ "//mojo/public/gles2" ]
-}