Use a direct include of strings headers in ui/, part 2.

ui/base/text/utf16_indexing.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 UI_BASE_TEXT_UTF16_INDEXING_H_
 #define UI_BASE_TEXT_UTF16_INDEXING_H_
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 #include "ui/base/ui_export.h"
 
 namespace ui {
 
 // Returns false if s[index-1] is a high surrogate and s[index] is a low
 // surrogate, true otherwise.
 UI_EXPORT bool IsValidCodePointIndex(const string16& s, size_t index);
 
 // |UTF16IndexToOffset| returns the number of code points between |base| and
 // |pos| in the given string. |UTF16OffsetToIndex| returns the index that is
@@ skipping 21 matching lines @@
 UI_EXPORT ptrdiff_t UTF16IndexToOffset(const string16& s,
                                        size_t base,
                                        size_t pos);
 UI_EXPORT size_t UTF16OffsetToIndex(const string16& s,
                                     size_t base,
                                     ptrdiff_t offset);
 
 }  // namespace ui
 
 #endif  // UI_BASE_TEXT_UTF16_INDEXING_H_