android: Introduce a ui::PlatformWindow implementation for android.

ui/platform_window/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("//build/config/ui.gni")
+
 source_set("platform_window") {
   sources = [
     "platform_window.h",
     "platform_window_delegate.h",
   ]
 
   deps = [
     "//base",
     "//ui/base",
     "//ui/gfx",
   ]
 }
+
+component("platform_impls") {
+  if (is_android) {
+    deps = [
+      "//ui/platform_window/android",
+      "//ui/platform_window/android:jni_headers",
+    ]
+  } else if (use_x11) {
+    deps = [
+      "//ui/platform_window/x11",
+    ]
+  } else if (is_win) {
+    deps = [
+      "//ui/platform_window/win",
+    ]
+  }
+}