Use JavaObjectWeakGlobalRef in ContentViewCore.

base/android/jni_helper.h

Index: base/android/jni_helper.h
diff --git a/base/android/jni_helper.h b/base/android/jni_helper.h
index 61359ef1725ff0fd0b4a983724336fcef1dc404d..bd3579fc89929ef1333b43b2a0b41cfff5be4810 100644
--- a/base/android/jni_helper.h
+++ b/base/android/jni_helper.h
@@ -10,6 +10,9 @@
 #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);
@@ -17,6 +20,8 @@ class JavaObjectWeakGlobalRef {
 
   base::android::ScopedJavaLocalRef<jobject> get(JNIEnv* env) const;
 
+  void reset();
+
  private:
   jweak obj_;
 
@@ -26,6 +31,6 @@ class 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_