Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/system/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_edk.gni")
 import("//testing/test.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
@@ skipping 88 matching lines @@
     "unique_identifier.cc",
     "unique_identifier.h",
     "waiter.cc",
     "waiter.h",
   ]
 
   mojo_edk_public_deps = [
     "mojo/edk/embedder",
     "mojo/edk/embedder:delegates",
     "mojo/edk/embedder:platform",
+    "mojo/edk/platform",
     "mojo/edk/util",
   ]
 
   mojo_sdk_public_deps = [
     "mojo/public/c/system",
     "mojo/public/cpp/system",
   ]
 
   deps = [
     "//base",
@@ skipping 76 matching lines @@
 
   mojo_edk_deps = [
     # TODO(vtl): Add separate test targets for this.
     "mojo/edk/util:perftests",
 
     "mojo/edk/system/test",
     "mojo/edk/system/test:perf",
     "mojo/edk/test:test_support",
   ]
 }