Have SearchProvider look up the omnibox start margin directly.

chrome/browser/ui/gtk/location_bar_view_gtk.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_UI_GTK_LOCATION_BAR_VIEW_GTK_H_
 #define CHROME_BROWSER_UI_GTK_LOCATION_BAR_VIEW_GTK_H_
 
 #include <gtk/gtk.h>
 
 #include <map>
@@ skipping 104 matching lines @@
                                     const GURL& alternate_nav_url) OVERRIDE;
   virtual void OnChanged() OVERRIDE;
   virtual void OnSelectionBoundsChanged() OVERRIDE;
   virtual void OnKillFocus() OVERRIDE;
   virtual void OnSetFocus() OVERRIDE;
   virtual void OnInputInProgress(bool in_progress) OVERRIDE;
   virtual gfx::Image GetFavicon() const OVERRIDE;
   virtual string16 GetTitle() const OVERRIDE;
   virtual InstantController* GetInstant() OVERRIDE;
   virtual content::WebContents* GetWebContents() const OVERRIDE;
-  virtual gfx::Rect GetOmniboxBounds() const OVERRIDE;
 
   // LocationBar:
   virtual void ShowFirstRunBubble() OVERRIDE;
   virtual string16 GetInputString() const OVERRIDE;
   virtual WindowOpenDisposition GetWindowOpenDisposition() const OVERRIDE;
   virtual content::PageTransition GetPageTransition() const OVERRIDE;
   virtual void AcceptInput() OVERRIDE;
   virtual void FocusLocation(bool select_all) OVERRIDE;
   virtual void FocusSearch() OVERRIDE;
   virtual void UpdateContentSettingsIcons() OVERRIDE;
@@ skipping 393 matching lines @@
   BooleanPrefMember edit_bookmarks_enabled_;
 
   // Used to remember the URL and title text when drag&drop has begun.
   GURL drag_url_;
   string16 drag_title_;
 
   DISALLOW_COPY_AND_ASSIGN(LocationBarViewGtk);
 };
 
 #endif  // CHROME_BROWSER_UI_GTK_LOCATION_BAR_VIEW_GTK_H_