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..87bc4f4c5cb3da2293ff05ba09208fb0f73a5ff8 100644
--- a/content/public/browser/browser_accessibility_state.h
+++ b/content/public/browser/browser_accessibility_state.h
@@ -27,6 +27,12 @@ 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