Add Chrome OS accessibility histograms.

content/public/browser/browser_accessibility_state.h

Index: content/public/browser/browser_accessibility_state.h
diff --git a/content/public/browser/browser_accessibility_state.h b/content/public/browser/browser_accessibility_state.h
index fdf4d6c21563ff704774059d5eaa25b3f8c84501..4c89f5aa6930afbb8eac7429c33982f6d84ef34b 100644
--- a/content/public/browser/browser_accessibility_state.h
+++ b/content/public/browser/browser_accessibility_state.h
@@ -5,6 +5,7 @@
 #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 {
@@ -27,6 +28,12 @@ class CONTENT_EXPORT BrowserAccessibilityState {
 
   // 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