Move nullable_string16.h to the string subdirectory.

content/browser/web_contents/web_contents_view_aura.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_contents_view_aura.h"
 
 #include "base/auto_reset.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/utf_string_conversions.h"
 #include "content/browser/renderer_host/dip_util.h"
@@ skipping 244 matching lines @@
                              pickle);
   }
 }
 
 // Utility to fill a WebDropData object from ui::OSExchangeData.
 void PrepareWebDropData(WebDropData* drop_data,
                         const ui::OSExchangeData& data) {
   string16 plain_text;
   data.GetString(&plain_text);
   if (!plain_text.empty())
-    drop_data->text = NullableString16(plain_text, false);
+    drop_data->text = base::NullableString16(plain_text, false);
 
   GURL url;
   string16 url_title;
   data.GetURLAndTitle(&url, &url_title);
   if (url.is_valid()) {
     drop_data->url = url;
     drop_data->url_title = url_title;
   }
 
   string16 html;
   GURL html_base_url;
   data.GetHtml(&html, &html_base_url);
   if (!html.empty())
-    drop_data->html = NullableString16(html, false);
+    drop_data->html = base::NullableString16(html, false);
   if (html_base_url.is_valid())
     drop_data->html_base_url = html_base_url;
 
   std::vector<ui::OSExchangeData::FileInfo> files;
   if (data.GetFilenames(&files) && !files.empty()) {
     for (std::vector<ui::OSExchangeData::FileInfo>::const_iterator
              it = files.begin(); it != files.end(); ++it) {
       drop_data->filenames.push_back(
           WebDropData::FileInfo(
               UTF8ToUTF16(it->path.AsUTF8Unsafe()),
@@ skipping 1127 matching lines @@
       event.location(),
       gfx::Screen::GetScreenFor(GetNativeView())->GetCursorScreenPoint(),
       ConvertAuraEventFlagsToWebInputEventModifiers(event.flags()));
   if (drag_dest_delegate_)
     drag_dest_delegate_->OnDrop();
   current_drop_data_.reset();
   return current_drag_op_;
 }
 
 }  // namespace content