Get rid of system_export.h, etc.

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.
 
 import("../../mojo_sdk.gni")
 
 mojo_sdk_source_set("system") {
   sources = [
     "system_thunks.c",
     "system_thunks.h",
   ]
-  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
   mojo_sdk_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.
 }
 
 # For internal use only.
 mojo_sdk_source_set("system_impl_private") {
   sources = [
     "system_impl_private_thunks.c",
     "system_impl_private_thunks.h",
   ]
-  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
   deps = [
     ":system_impl_private_api",
   ]
   mojo_sdk_deps = [ "mojo/public/c/system" ]
 }
 
 # For internal use only.
 mojo_sdk_source_set("system_impl_private_api") {
   sources = [
     "system_impl_private.h",
@@ skipping 78 matching lines @@
 }
 
 mojo_sdk_source_set("mgl_onscreen_thunks") {
   sources = [
     "mgl_onscreen_thunks.c",
     "mgl_onscreen_thunks.h",
   ]
 
   mojo_sdk_deps = [ "mojo/public/c/gpu:MGL_onscreen" ]
 }