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

sky/shell/gpu/ganesh_surface.cc

Index: sky/shell/gpu/ganesh_surface.cc
diff --git a/sky/shell/gpu/ganesh_surface.cc b/sky/shell/gpu/ganesh_surface.cc
index c9771de53b09331c55d511f186b2dea7b3555c0b..e64853f8e83d80a5efb6b134c80456f9cbf6c2a4 100644
--- a/sky/shell/gpu/ganesh_surface.cc
+++ b/sky/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);