Fix debug build GTK assert in TabContentsDragSource::OnDragBegin().

chrome/browser/gtk/tab_contents_drag_source.cc

 // Copyright (c) 2009 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 "chrome/browser/gtk/tab_contents_drag_source.h"
 
 #include <string>
 
 #include "app/gtk_dnd_util.h"
 #include "base/file_util.h"
@@ skipping 301 matching lines @@
                         reinterpret_cast<const guchar*>(
                             generated_download_file_name.value().c_str()),
                         generated_download_file_name.value().length());
   }
 
   if (drag_pixbuf_) {
     gtk_widget_set_size_request(drag_icon_,
                                 gdk_pixbuf_get_width(drag_pixbuf_),
                                 gdk_pixbuf_get_height(drag_pixbuf_));
 
-    GdkScreen* screen = gtk_widget_get_screen(drag_icon_);
-    GdkColormap* rgba = gdk_screen_get_rgba_colormap(screen);
-    if (rgba)
-      gtk_widget_set_colormap(drag_icon_, rgba);
+    // We only need to do this once.
+    if (!GTK_WIDGET_REALIZED(drag_icon_)) {
+      GdkScreen* screen = gtk_widget_get_screen(drag_icon_);
+      GdkColormap* rgba = gdk_screen_get_rgba_colormap(screen);
+      if (rgba)
+        gtk_widget_set_colormap(drag_icon_, rgba);
+    }
 
     gtk_drag_set_icon_widget(drag_context, drag_icon_,
                              image_offset_.x(), image_offset_.y());
   }
 }
 
 void TabContentsDragSource::OnDragEnd(GtkWidget* sender,
                                       GdkDragContext* drag_context) {
   if (drag_pixbuf_) {
     g_object_unref(drag_pixbuf_);
@@ skipping 34 matching lines @@
   cairo_t* cr = gdk_cairo_create(event->window);
   gdk_cairo_rectangle(cr, &event->area);
   cairo_clip(cr);
   cairo_set_operator(cr, CAIRO_OPERATOR_SOURCE);
   gdk_cairo_set_source_pixbuf(cr, drag_pixbuf_, 0, 0);
   cairo_paint(cr);
   cairo_destroy(cr);
 
   return TRUE;
 }