WebVR: Don't crash when receiving null image from rendering context.

third_party/WebKit/Source/modules/vr/VRDisplay.cpp

 // Copyright 2016 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 "modules/vr/VRDisplay.h"
 
 #include "core/css/StylePropertySet.h"
 #include "core/dom/DOMException.h"
 #include "core/dom/FrameRequestCallback.h"
 #include "core/dom/ScriptedAnimationController.h"
@@ skipping 523 matching lines @@
     }
   }
 
   RefPtr<Image> image_ref = rendering_context_->GetImage(
       kPreferAcceleration, kSnapshotReasonCreateImageBitmap);
 
   // Hardware-accelerated rendering should always be texture backed,
   // as implemented by AcceleratedStaticBitmapImage. Ensure this is
   // the case, don't attempt to render if using an unexpected drawing
   // path.
-  if (!image_ref->IsTextureBacked()) {
+  if (!image_ref.Get() || !image_ref->IsTextureBacked()) {
     NOTREACHED() << "WebVR requires hardware-accelerated rendering to texture";
     return;
   }
 
   // The AcceleratedStaticBitmapImage must be kept alive until the
   // mailbox is used via createAndConsumeTextureCHROMIUM, the mailbox
   // itself does not keep it alive. We must keep a reference to the
   // image until the mailbox was consumed.
   StaticBitmapImage* static_image =
       static_cast<StaticBitmapImage*>(image_ref.Get());
@@ skipping 258 matching lines @@
   visitor->Trace(stage_parameters_);
   visitor->Trace(eye_parameters_left_);
   visitor->Trace(eye_parameters_right_);
   visitor->Trace(layer_);
   visitor->Trace(rendering_context_);
   visitor->Trace(scripted_animation_controller_);
   visitor->Trace(pending_present_resolvers_);
 }
 
 }  // namespace blink