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

ui/gl/gl_surface_ios.mm

 // Copyright 2014 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 "base/mac/scoped_nsautorelease_pool.h"
 #include "ui/gl/gl_surface_ios.h"
 #include "ui/gl/gl_context.h"
 #include "ui/gl/gl_enums.h"
 #include "base/logging.h"
 
@@ skipping 200 matching lines @@
   };
 
   framebuffer_setup_complete_ = true;
 }
 
 bool GLSurfaceIOS::SwapBuffers() {
   glBindRenderbuffer(GL_RENDERBUFFER, colorbuffer_);
   return [[EAGLContext currentContext] presentRenderbuffer:GL_RENDERBUFFER];
 }
 
+unsigned int GLSurfaceIOS::GetBackingFrameBufferObject() {
+  return framebuffer_;
+}
+
 void GLSurfaceIOS::Destroy() {
   DCHECK(glGetError() == GL_NO_ERROR);
 
   glDeleteFramebuffers(1, &framebuffer_);
   glDeleteRenderbuffers(1, &colorbuffer_);
   // Deletes on GL_NONEs are ignored
   glDeleteRenderbuffers(1, &depthbuffer_);
   glDeleteRenderbuffers(1, &stencilbuffer_);
   glDeleteRenderbuffers(1, &depth_stencil_packed_buffer_);
 
@@ skipping 28 matching lines @@
   scoped_refptr<GLSurfaceIOS> surface =
     new GLSurfaceIOS(window, requested_configuration);
 
   if (!surface->Initialize())
     return NULL;
 
   return surface;
 }
 
 }  // namespace gfx