Use JavaObjectWeakGlobalRef in ContentViewCore.

base/android/jni_helper.h

 // Copyright (c) 2012 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.
 
 #ifndef BASE_ANDROID_JNI_HELPER_H_
 #define BASE_ANDROID_JNI_HELPER_H_
 
 #include <jni.h>
 
 #include "base/android/scoped_java_ref.h"
 
 // Manages WeakGlobalRef lifecycle.
+// This class is not thread-safe w.r.t. get() and reset(). Multiple threads may
+// safely use get() concurrently, but if the user calls reset() (or of course,
+// calls the destructor) they'll need to provide their own synchronization.
 class JavaObjectWeakGlobalRef {
  public:
   JavaObjectWeakGlobalRef(JNIEnv* env, jobject obj);
   virtual ~JavaObjectWeakGlobalRef();
 
   base::android::ScopedJavaLocalRef<jobject> get(JNIEnv* env) const;
 
+  void reset();
+
  private:
   jweak obj_;
 
   DISALLOW_COPY_AND_ASSIGN(JavaObjectWeakGlobalRef);
 };
 
 // Get the real object stored in the weak reference returned as a
 // ScopedJavaLocalRef.
 base::android::ScopedJavaLocalRef<jobject> GetRealObject(
-    JNIEnv* env, jobject obj);
+    JNIEnv* env, jweak obj);
 
 #endif  // BASE_ANDROID_JNI_HELPER_H_