Move nullable_string16.h to the string subdirectory.

content/browser/web_contents/web_drag_dest_win.cc

 // 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.
 
 #include "content/browser/web_contents/web_drag_dest_win.h"
 
 #include <windows.h>
 #include <shlobj.h>
 
 #include "base/win/win_util.h"
@@ skipping 60 matching lines @@
       drop_data->url = test_url;
   }
   std::vector<base::string16> filenames;
   ui::ClipboardUtil::GetFilenames(data_object, &filenames);
   for (size_t i = 0; i < filenames.size(); ++i)
     drop_data->filenames.push_back(
         WebDropData::FileInfo(filenames[i], base::string16()));
   base::string16 text;
   ui::ClipboardUtil::GetPlainText(data_object, &text);
   if (!text.empty()) {
-    drop_data->text = NullableString16(text, false);
+    drop_data->text = base::NullableString16(text, false);
   }
   base::string16 html;
   std::string html_base_url;
   ui::ClipboardUtil::GetHtml(data_object, &html, &html_base_url);
   if (!html.empty()) {
-    drop_data->html = NullableString16(html, false);
+    drop_data->html = base::NullableString16(html, false);
   }
   if (!html_base_url.empty()) {
     drop_data->html_base_url = GURL(html_base_url);
   }
   ui::ClipboardUtil::GetWebCustomData(data_object, &drop_data->custom_data);
 }
 
 }  // namespace
 
 // InterstitialDropTarget is like a ui::DropTargetWin implementation that
@@ skipping 179 matching lines @@
   // This isn't always correct, but at least it's a close approximation.
   // For now, we always map a move to a copy to prevent potential data loss.
   DWORD drop_effect = WebDragOpToWinDragOp(drag_cursor_);
   DWORD result = drop_effect != DROPEFFECT_MOVE ? drop_effect : DROPEFFECT_COPY;
 
   drop_data_.reset();
   return result;
 }
 
 }  // namespace content