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

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

Index: third_party/WebKit/Source/modules/vr/VRDisplay.cpp
diff --git a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
index 27228f4c3112e7f043355473a92f84f67ce20b10..b3e54313040e86fb906b7ae6543483f98d3528bc 100644
--- a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
+++ b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
@@ -559,7 +559,7 @@ void VRDisplay::submitFrame() {
   // 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;
   }