tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/unique_vector.h

 // Copyright 2014 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 TOOLS_GN_UNIQUE_VECTOR_H_
 #define TOOLS_GN_UNIQUE_VECTOR_H_
 
 #include <algorithm>
 
 #include "base/containers/hash_tables.h"
@@ skipping 15 matching lines @@
 class UniquifyRef {
  public:
   UniquifyRef()
       : value_(nullptr),
         vect_(nullptr),
         index_(static_cast<size_t>(-1)),
         hash_val_(0) {
   }
 
   // Initialize with a pointer to a value.
-  UniquifyRef(const T* v)
+  explicit UniquifyRef(const T* v)
       : value_(v),
         vect_(nullptr),
         index_(static_cast<size_t>(-1)) {
     FillHashValue();
   }
 
   // Initialize with an array + index.
   UniquifyRef(const std::vector<T>* v, size_t i)
       : value_(nullptr),
         vect_(v),
@@ skipping 120 matching lines @@
 
  private:
   typedef internal::UniquifyRef<T> Ref;
   typedef base::hash_set<Ref> HashSet;
 
   HashSet set_;
   Vector vector_;
 };
 
 #endif  // TOOLS_GN_UNIQUE_VECTOR_H_