Add an explicit bool conversion to scoped_refptr<T>.

base/memory/ref_counted.h

Index: base/memory/ref_counted.h
diff --git a/base/memory/ref_counted.h b/base/memory/ref_counted.h
index be4919c1a51908699c260936f4f6b3c62a98d9ab..4eab2dda5c0591a8e19a0f1ae38fafe5250b3c1c 100644
--- a/base/memory/ref_counted.h
+++ b/base/memory/ref_counted.h
@@ -303,13 +303,15 @@ class scoped_refptr {
   operator T*() const { return ptr_; }
 #endif
 
+  explicit operator bool() const { return ptr_ != nullptr; }
+
   T& operator*() const {
-    assert(ptr_ != NULL);
+    assert(ptr_ != nullptr);
     return *ptr_;
   }
 
   T* operator->() const {
-    assert(ptr_ != NULL);
+    assert(ptr_ != nullptr);
     return ptr_;
   }