Fix offscreen rendering on iOS by not assuming that the default window bound framebuffer is 0

sky/shell/gpu/ganesh_surface.cc

 // 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 "sky/shell/gpu/ganesh_surface.h"
 
 #include "base/logging.h"
 #include "third_party/skia/include/gpu/GrContext.h"
 
 namespace sky {
 namespace shell {
 
-GaneshSurface::GaneshSurface(GaneshContext* context, const gfx::Size& size) {
+GaneshSurface::GaneshSurface(intptr_t window_fbo,
+              GaneshContext* context,
+              const gfx::Size& size) {
   GrBackendRenderTargetDesc desc;
   desc.fWidth = size.width();
   desc.fHeight = size.height();
   desc.fConfig = kSkia8888_GrPixelConfig;
   desc.fOrigin = kBottomLeft_GrSurfaceOrigin;
+  desc.fRenderTargetHandle = window_fbo;
 
   auto target = skia::AdoptRef(context->gr()->wrapBackendRenderTarget(desc));
   DCHECK(target);
   surface_ = skia::AdoptRef(SkSurface::NewRenderTargetDirect(target.get()));
   DCHECK(surface_);
 }
 
 GaneshSurface::~GaneshSurface() {
 }
 
 }  // namespace shell
 }  // namespace sky