Refactor SkyShell to allow multiple SkyViews

sky/shell/ios/platform_view_ios.mm

 // Copyright 2015 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.
 
 #include "platform_view_ios.h"
 
 namespace sky {
 namespace shell {
 
+PlatformView* PlatformView::Create(const Config& config) {
+  return new PlatformViewIOS(config);
+}
+
 void PlatformViewIOS::SurfaceCreated(gfx::AcceleratedWidget widget) {
   DCHECK(window_ == 0);
   window_ = widget;
   SurfaceWasCreated();
 }
 
 void PlatformViewIOS::SurfaceDestroyed() {
   DCHECK(window_);
   window_ = 0;
   SurfaceWasDestroyed();
 }
 
 }  // namespace shell
 }  // namespace sky