Add InitWidgets() phase for GTK infobars.

chrome/browser/ui/gtk/infobars/infobar_gtk.h

Index: chrome/browser/ui/gtk/infobars/infobar_gtk.h
===================================================================
--- chrome/browser/ui/gtk/infobars/infobar_gtk.h	(revision 194480)
+++ chrome/browser/ui/gtk/infobars/infobar_gtk.h	(working copy)
@@ -98,6 +98,17 @@
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
+  // Called the first time we try to show the infobar.  Inits any widgets and
+  // related objects necessary.
+  //
+  // NOTE: Subclasses who need to init widgets should override this function and
+  // explicitly call their parent's implementation first, then continue with
+  // further work they need to do.  Failing to call the parent implementation
+  // first (or at all), or setting up widgets in the constructor instead of
+  // here, will lead to bad side effects like crashing or having this function
+  // get called repeatedly.
+  virtual void InitWidgets();
+
   // The second highest widget in the hierarchy (after the |widget_|).
   GtkWidget* bg_box_;