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

BUG=
R=iansf@google.com

Review URL: https://codereview.chromium.org/1183213007.

shell/gpu/ganesh_surface.cc

@@ -10,12 +10,15 @@
 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);

shell/gpu/ganesh_surface.h

@@ -15,11 +15,13 @@ namespace sky {
 namespace shell {
 
 // GaneshSurface holds an SkSurface configured to render with Ganesh. Using the
-// provided GaneshContext, GaneshSurface wraps an SkSurface around FBO 0 so that
-// you can use |canvas()| to draw to FBO 0.
+// provided GaneshContext, GaneshSurface wraps an SkSurface around the window
+// bound FBO so that you can use |canvas()| to draw to that window bound FBO.
 class GaneshSurface {
  public:
-  GaneshSurface(GaneshContext* context, const gfx::Size& size);
+  GaneshSurface(intptr_t window_fbo,
+                GaneshContext* context,
+                const gfx::Size& size);
   ~GaneshSurface();
 
   SkCanvas* canvas() const { return surface_->getCanvas(); }

shell/gpu/rasterizer.cc

@@ -54,7 +54,7 @@ void Rasterizer::Draw(skia::RefPtr<SkPicture> picture) {
 
   EnsureGLContext();
   CHECK(context_->MakeCurrent(surface_.get()));
-  EnsureGaneshSurface(size);
+  EnsureGaneshSurface(surface_->GetBackingFrameBufferObject(), size);
 
   DrawPicture(picture.get());
   surface_->SwapBuffers();
@@ -84,9 +84,11 @@ void Rasterizer::EnsureGLContext() {
   ganesh_context_.reset(new GaneshContext(context_.get()));
 }
 
-void Rasterizer::EnsureGaneshSurface(const gfx::Size& size) {
+void Rasterizer::EnsureGaneshSurface(intptr_t window_fbo,
+                                     const gfx::Size& size) {
   if (!ganesh_surface_ || ganesh_surface_->size() != size)
-    ganesh_surface_.reset(new GaneshSurface(ganesh_context_.get(), size));
+    ganesh_surface_.reset(
+      new GaneshSurface(window_fbo, ganesh_context_.get(), size));
 }
 
 }  // namespace shell

shell/gpu/rasterizer.h

@@ -38,7 +38,7 @@ class Rasterizer : public GPUDelegate {
 
  private:
   void EnsureGLContext();
-  void EnsureGaneshSurface(const gfx::Size& size);
+  void EnsureGaneshSurface(intptr_t window_fbo, const gfx::Size& size);
   void DrawPicture(SkPicture* picture);
 
   scoped_refptr<gfx::GLShareGroup> share_group_;