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

ui/gl/gl_surface_ios.mm

Index: ui/gl/gl_surface_ios.mm
diff --git a/ui/gl/gl_surface_ios.mm b/ui/gl/gl_surface_ios.mm
index 669e7e2b99daf40bc508a8feb00d972dde1059c1..a65c666ab89b76d0d330fac7bc84fa2e9d502ec9 100644
--- a/ui/gl/gl_surface_ios.mm
+++ b/ui/gl/gl_surface_ios.mm
@@ -218,6 +218,10 @@ bool GLSurfaceIOS::SwapBuffers() {
   return [[EAGLContext currentContext] presentRenderbuffer:GL_RENDERBUFFER];
 }
 
+unsigned int GLSurfaceIOS::GetBackingFrameBufferObject() {
+  return framebuffer_;
+}
+
 void GLSurfaceIOS::Destroy() {
   DCHECK(glGetError() == GL_NO_ERROR);