Convert string16 to base::string16 in content.

content/browser/renderer_host/render_widget_host_view_guest.cc

Index: content/browser/renderer_host/render_widget_host_view_guest.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_guest.cc b/content/browser/renderer_host/render_widget_host_view_guest.cc
index 629938495b75b886df3c337b0901bcf4784aa547..49562c8dad7e4a074555919d0bfdbbee6bea241d 100644
--- a/content/browser/renderer_host/render_widget_host_view_guest.cc
+++ b/content/browser/renderer_host/render_widget_host_view_guest.cc
@@ -169,7 +169,8 @@ void RenderWidgetHostViewGuest::Destroy() {
   platform_view_->Destroy();
 }
 
-void RenderWidgetHostViewGuest::SetTooltipText(const string16& tooltip_text) {
+void RenderWidgetHostViewGuest::SetTooltipText(
+    const base::string16& tooltip_text) {
   platform_view_->SetTooltipText(tooltip_text);
 }
 
@@ -324,7 +325,7 @@ void RenderWidgetHostViewGuest::DidUpdateBackingStore(
   NOTREACHED();
 }
 
-void RenderWidgetHostViewGuest::SelectionChanged(const string16& text,
+void RenderWidgetHostViewGuest::SelectionChanged(const base::string16& text,
                                                  size_t offset,
                                                  const gfx::Range& range) {
   platform_view_->SelectionChanged(text, offset, range);