Move nullable_string16.h to the string subdirectory.

content/browser/web_contents/web_drag_dest_gtk.cc

Index: content/browser/web_contents/web_drag_dest_gtk.cc
diff --git a/content/browser/web_contents/web_drag_dest_gtk.cc b/content/browser/web_contents/web_drag_dest_gtk.cc
index d34526e0e580c9151cd63f5a100a6312fd22ea80..ef63ba993fbf735e8d528ed835adc5af27d08800 100644
--- a/content/browser/web_contents/web_drag_dest_gtk.cc
+++ b/content/browser/web_contents/web_drag_dest_gtk.cc
@@ -185,7 +185,7 @@ void WebDragDestGtk::OnDragDataReceived(
     if (target == ui::GetAtomForTarget(ui::TEXT_PLAIN)) {
       guchar* text = gtk_selection_data_get_text(data);
       if (text) {
-        drop_data_->text = NullableString16(
+        drop_data_->text = base::NullableString16(
             UTF8ToUTF16(std::string(reinterpret_cast<const char*>(text))),
             false);
         g_free(text);
@@ -209,7 +209,7 @@ void WebDragDestGtk::OnDragDataReceived(
             // This is a hack. Some file managers also populate text/plain with
             // a file URL when dragging files, so we clear it to avoid exposing
             // it to the web content.
-            drop_data_->text = NullableString16(true);
+            drop_data_->text = base::NullableString16(true);
           } else if (!drop_data_->url.is_valid()) {
             // Also set the first non-file URL as the URL content for the drop.
             drop_data_->url = url;
@@ -219,7 +219,7 @@ void WebDragDestGtk::OnDragDataReceived(
       }
     } else if (target == ui::GetAtomForTarget(ui::TEXT_HTML)) {
       // TODO(estade): Can the html have a non-UTF8 encoding?
-      drop_data_->html = NullableString16(
+      drop_data_->html = base::NullableString16(
           UTF8ToUTF16(std::string(reinterpret_cast<const char*>(raw_data),
                                   data_length)),
           false);