Fix possible memory leak due to tooltip property in aura::Window

content/browser/renderer_host/render_widget_host_view_aura.h

Index: content/browser/renderer_host/render_widget_host_view_aura.h
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.h b/content/browser/renderer_host/render_widget_host_view_aura.h
index 2f0d585822a97af6bc5c9e43d2634cf5bcedc1ab..f982fdc9535cda92ea71d24e92e255d1de72980a 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.h
+++ b/content/browser/renderer_host/render_widget_host_view_aura.h
@@ -143,6 +143,9 @@ class RenderWidgetHostViewAura : public RenderWidgetHostView,
   // removed from the list on an ET_TOUCH_RELEASED event.
   WebKit::WebTouchEvent touch_event_;
 
+  // Current tooltip text.
+  string16 tooltip_;
+
 #if defined(UI_COMPOSITOR_IMAGE_TRANSPORT)
   std::vector< base::Callback<void(void)> > on_compositing_ended_callbacks_;