Adds new experimental accessibility extension api to enable or disable native accessibility.

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 16cf706d3867dbd6980bd406224fa9fc4ddca612..8e18fe8cb36aca2db6476bef309f16cb144d25ba 100644
--- a/content/public/browser/browser_accessibility_state.h
+++ b/content/public/browser/browser_accessibility_state.h
@@ -6,6 +6,7 @@
 #define CONTENT_PUBLIC_BROWSER_BROWSER_ACCESSIBILITY_STATE_H_
 
 #include "base/callback_forward.h"
+
 #include "content/common/content_export.h"
 
 namespace content {
@@ -23,6 +24,9 @@ class CONTENT_EXPORT BrowserAccessibilityState {
   // Called when accessibility is enabled manually (via command-line flag).
   virtual void OnAccessibilityEnabledManually() = 0;
 
+  // Called when an extension takes over responsibility for web accessibility.
+  virtual void OnExtensionWebAccessibilityEnabled() = 0;
+
   // Called when screen reader client is detected.
   virtual void OnScreenReaderDetected() = 0;