Update from https://crrev.com/306655

base/memory/ref_counted.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_MEMORY_REF_COUNTED_H_
 #define BASE_MEMORY_REF_COUNTED_H_
 
 #include <cassert>
 #include <iosfwd>
 
 #include "base/atomic_ref_count.h"
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #ifndef NDEBUG
 #include "base/logging.h"
 #endif
 #include "base/threading/thread_collision_warner.h"
 #include "build/build_config.h"
 
-#if defined(OS_LINUX) || defined(OS_MACOSX) || defined(OS_IOS) || defined(OS_ANDROID)
-#define DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR
-#endif
-
 namespace base {
 
 namespace subtle {
 
 class BASE_EXPORT RefCountedBase {
  public:
   bool HasOneRef() const { return ref_count_ == 1; }
 
  protected:
   RefCountedBase()
@@ skipping 256 matching lines @@
       AddRef(ptr_);
   }
 
   ~scoped_refptr() {
     if (ptr_)
       Release(ptr_);
   }
 
   T* get() const { return ptr_; }
 
-#if !defined(DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR)
-  // Allow scoped_refptr<C> to be used in boolean expression
-  // and comparison operations.
-  operator T*() const { return ptr_; }
-#endif
-
   T& operator*() const {
     assert(ptr_ != NULL);
     return *ptr_;
   }
 
   T* operator->() const {
     assert(ptr_ != NULL);
     return ptr_;
   }
 
@@ skipping 20 matching lines @@
   void swap(T** pp) {
     T* p = ptr_;
     ptr_ = *pp;
     *pp = p;
   }
 
   void swap(scoped_refptr<T>& r) {
     swap(&r.ptr_);
   }
 
-#if defined(DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR)
+ private:
+  // Allow scoped_refptr<T> to be used in boolean expressions, but not
+  // implicitly convertible to a real bool (which is dangerous).
+  //
+  // Note that this trick is only safe when the == and != operators
+  // are declared explicitly, as otherwise "refptr1 == refptr2"
+  // will compile but do the wrong thing (i.e., convert to Testable
+  // and then do the comparison).
+  typedef T* scoped_refptr::*Testable;
+
+ public:
+  operator Testable() const { return ptr_ ? &scoped_refptr::ptr_ : nullptr; }
+
   template <typename U>
   bool operator==(const scoped_refptr<U>& rhs) const {
     return ptr_ == rhs.get();
   }
 
   template <typename U>
   bool operator!=(const scoped_refptr<U>& rhs) const {
     return !operator==(rhs);
   }
 
   template <typename U>
   bool operator<(const scoped_refptr<U>& rhs) const {
     return ptr_ < rhs.get();
   }
-#endif
 
  protected:
   T* ptr_;
 
  private:
   // Non-inline helpers to allow:
   //     class Opaque;
   //     extern template class scoped_refptr<Opaque>;
   // Otherwise the compiler will complain that Opaque is an incomplete type.
   static void AddRef(T* ptr);
@@ skipping 10 matching lines @@
   ptr->Release();
 }
 
 // Handy utility for creating a scoped_refptr<T> out of a T* explicitly without
 // having to retype all the template arguments
 template <typename T>
 scoped_refptr<T> make_scoped_refptr(T* t) {
   return scoped_refptr<T>(t);
 }
 
-#if defined(DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR)
-// Temporary operator overloads to facilitate the transition...
+// Temporary operator overloads to facilitate the transition. See
+// https://crbug.com/110610.
 template <typename T, typename U>
 bool operator==(const scoped_refptr<T>& lhs, const U* rhs) {
   return lhs.get() == rhs;
 }
 
 template <typename T, typename U>
 bool operator==(const T* lhs, const scoped_refptr<U>& rhs) {
   return lhs == rhs.get();
 }
 
 template <typename T, typename U>
 bool operator!=(const scoped_refptr<T>& lhs, const U* rhs) {
   return !operator==(lhs, rhs);
 }
 
 template <typename T, typename U>
 bool operator!=(const T* lhs, const scoped_refptr<U>& rhs) {
   return !operator==(lhs, rhs);
 }
 
 template <typename T>
 std::ostream& operator<<(std::ostream& out, const scoped_refptr<T>& p) {
   return out << p.get();
 }
-#endif  // defined(DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR)
 
 #endif  // BASE_MEMORY_REF_COUNTED_H_