Prevent the browser from crashing when exiting pages that use WebVR.

chrome/browser/android/vr_shell/vr_shell_delegate.cc

 // 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 "chrome/browser/android/vr_shell/vr_shell_delegate.h"
 
 #include "base/android/jni_android.h"
 #include "chrome/browser/android/vr_shell/vr_shell.h"
 #include "device/vr/android/gvr/gvr_device_provider.h"
 #include "jni/VrShellDelegate_jni.h"
@@ skipping 86 matching lines @@
     if (!context)
       return nullptr;
 
     non_presenting_delegate_.reset(new GvrNonPresentingDelegate(context));
   }
   return static_cast<GvrNonPresentingDelegate*>(non_presenting_delegate_.get())
       ->GetWeakPtr();
 }
 
 void VrShellDelegate::DestroyNonPresentingDelegate() {
-  non_presenting_delegate_.reset(nullptr);
-  JNIEnv* env = AttachCurrentThread();
-  Java_VrShellDelegate_shutdownNonPresentingNativeContext(
-      env, j_vr_shell_delegate_.obj());
+  if (non_presenting_delegate_) {
+    non_presenting_delegate_.reset(nullptr);
+    JNIEnv* env = AttachCurrentThread();
+    Java_VrShellDelegate_shutdownNonPresentingNativeContext(
+        env, j_vr_shell_delegate_.obj());
+  }
 }
 
 // ----------------------------------------------------------------------------
 // Native JNI methods
 // ----------------------------------------------------------------------------
 
 bool RegisterVrShellDelegate(JNIEnv* env) {
   return RegisterNativesImpl(env);
 }
 
 jlong Init(JNIEnv* env, const JavaParamRef<jobject>& obj) {
   return reinterpret_cast<intptr_t>(new VrShellDelegate(env, obj));
 }
 
 }  // namespace vr_shell