Add a std::nullptr_t constructor to base::WeakPtr.

base/memory/weak_ptr.h

Index: base/memory/weak_ptr.h
diff --git a/base/memory/weak_ptr.h b/base/memory/weak_ptr.h
index 006e1fd33fdf7c1585b19b1ddb7dfeb5a245692e..3b8bcb1b077607f36224fa52837e72161f3ed295 100644
--- a/base/memory/weak_ptr.h
+++ b/base/memory/weak_ptr.h
@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 // Weak pointers are pointers to an object that do not affect its lifetime,
-// and which may be invalidated (i.e. reset to NULL) by the object, or its
+// and which may be invalidated (i.e. reset to nullptr) by the object, or its
 // owner, at any time, most commonly when the object is about to be deleted.
 
 // Weak pointers are useful when an object needs to be accessed safely by one
@@ -70,6 +70,7 @@
 #ifndef BASE_MEMORY_WEAK_PTR_H_
 #define BASE_MEMORY_WEAK_PTR_H_
 
+#include <cstddef>
 #include <type_traits>
 
 #include "base/base_export.h"
@@ -199,8 +200,9 @@ template <typename T> class WeakPtrFactory;
 template <typename T>
 class WeakPtr : public internal::WeakPtrBase {
  public:
-  WeakPtr() : ptr_(NULL) {
-  }
+  WeakPtr() : ptr_(nullptr) {}
+
+  WeakPtr(std::nullptr_t) : ptr_(nullptr) {}
 
   // Allow conversion from U to T provided U "is a" T. Note that this
   // is separate from the (implicit) copy constructor.
@@ -208,20 +210,20 @@ class WeakPtr : public internal::WeakPtrBase {
   WeakPtr(const WeakPtr<U>& other) : WeakPtrBase(other), ptr_(other.ptr_) {
   }
 
-  T* get() const { return ref_.is_valid() ? ptr_ : NULL; }
+  T* get() const { return ref_.is_valid() ? ptr_ : nullptr; }
 
   T& operator*() const {
-    DCHECK(get() != NULL);
+    DCHECK(get() != nullptr);
     return *get();
   }
   T* operator->() const {
-    DCHECK(get() != NULL);
+    DCHECK(get() != nullptr);
     return get();
   }
 
   void reset() {
     ref_ = internal::WeakReference();
-    ptr_ = NULL;
+    ptr_ = nullptr;
   }
 
   // Implement "Safe Bool Idiom"
@@ -246,7 +248,7 @@ class WeakPtr : public internal::WeakPtrBase {
   typedef T* WeakPtr::*Testable;
 
  public:
-  operator Testable() const { return get() ? &WeakPtr::ptr_ : NULL; }
+  operator Testable() const { return get() ? &WeakPtr::ptr_ : nullptr; }
 
  private:
   // Explicitly declare comparison operators as required by the "Safe Bool
@@ -265,7 +267,7 @@ class WeakPtr : public internal::WeakPtrBase {
   }
 
   // This pointer is only valid when ref_.is_valid() is true.  Otherwise, its
-  // value is undefined (as opposed to NULL).
+  // value is undefined (as opposed to nullptr).
   T* ptr_;
 };
 
@@ -280,9 +282,7 @@ class WeakPtrFactory {
   explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
   }
 
-  ~WeakPtrFactory() {
-    ptr_ = NULL;
-  }
+  ~WeakPtrFactory() { ptr_ = nullptr; }
 
   WeakPtr<T> GetWeakPtr() {
     DCHECK(ptr_);