Add Chrome OS accessibility histograms.

content/public/browser/browser_accessibility_state.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 CONTENT_PUBLIC_BROWSER_BROWSER_ACCESSIBILITY_STATE_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_ACCESSIBILITY_STATE_H_
 
+#include "base/callback_forward.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 // The BrowserAccessibilityState class is used to determine if the browser
 // should be customized for users with assistive technology, such as screen
 // readers.
 class CONTENT_EXPORT BrowserAccessibilityState {
  public:
   virtual ~BrowserAccessibilityState() { }
 
   // Returns the singleton instance.
   static BrowserAccessibilityState* GetInstance();
 
   // Called when accessibility is enabled manually (via command-line flag).
   virtual void OnAccessibilityEnabledManually() = 0;
 
   // Called when screen reader client is detected.
   virtual void OnScreenReaderDetected() = 0;
 
   // Returns true if the browser should be customized for accessibility.
   virtual bool IsAccessibleBrowser() = 0;
+
+  // Add a callback method that will be called once, a small while after the
+  // browser starts up, when accessibility state histograms are updated.
+  // Use this to register a method to update additional accessibility
+  // histograms.
+  virtual void AddHistogramCallback(base::Closure callback) = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_ACCESSIBILITY_STATE_H_