[Reland] Mojo: Eliminate duplicate C API symbols

mojo/public/platform/native/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.
-
-assert(!defined(is_nacl) || !is_nacl)
-
-source_set("system") {
-  sources = [
-    "system_thunks.cc",
-    "system_thunks.h",
-  ]
-  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
-  deps = [
-    "//mojo/public/c/system",
-  ]
-
-  # The GYP target analogous to this one builds this code into a
-  # static library.  When building for Android, both the GYP and GN
-  # builds add --exclude-libs=ALL globally, which means that all
-  # symbols in static libraries are excluded from export.  That's a
-  # problem, as code outside this target needs to be able to call
-  # MojoSetSystemThunks(). Therefore, the GYP target needs to specifiy
-  # that all dependent targets remove that link flag. Since GN uses a
-  # source_set here, this flag change is not needed.
-}
-
-source_set("gpu_thunks") {
-  sources = []
-}