Allow nullptr to be converted to JavaRef subclasses.

base/android/scoped_java_ref.h

Index: base/android/scoped_java_ref.h
diff --git a/base/android/scoped_java_ref.h b/base/android/scoped_java_ref.h
index cf8edc984626f8fd61564d876b137980f8b2d5f1..c844c8d1e983e44b768980deff0ef4fc602d687d 100644
--- a/base/android/scoped_java_ref.h
+++ b/base/android/scoped_java_ref.h
@@ -148,6 +148,7 @@ template<typename T>
 class ScopedJavaLocalRef : public JavaRef<T> {
  public:
   ScopedJavaLocalRef() : env_(nullptr) {}
+  ScopedJavaLocalRef(std::nullptr_t) : env_(nullptr) {}
 
   // Non-explicit copy constructor, to allow ScopedJavaLocalRef to be returned
   // by value as this is the normal usage pattern.
@@ -238,6 +239,7 @@ template<typename T>
 class ScopedJavaGlobalRef : public JavaRef<T> {
  public:
   ScopedJavaGlobalRef() {}
+  ScopedJavaGlobalRef(std::nullptr_t) {}
 
   ScopedJavaGlobalRef(const ScopedJavaGlobalRef<T>& other) {
     this->Reset(other);