Add a11y on-screen keyboard to accessibility menu on system tray.

ash/accessibility_delegate.h

Index: ash/accessibility_delegate.h
diff --git a/ash/accessibility_delegate.h b/ash/accessibility_delegate.h
index 8d4b9328edff50da4af919c2e8da604c301ab6dc..037a40f83024aa468047e776eb4aa66130e206b9 100644
--- a/ash/accessibility_delegate.h
+++ b/ash/accessibility_delegate.h
@@ -63,6 +63,9 @@ class ASH_EXPORT AccessibilityDelegate {
   // Returns if autoclick is enabled or not.
   virtual bool IsAutoclickEnabled() const = 0;
 
+  // Invoked to enable or disable the a11y on-screen keyboard.
+  virtual void SetVirtualKeyboardEnabled(bool enabled) = 0;
+
   // Returns if the a11y virtual keyboard is enabled.
   virtual bool IsVirtualKeyboardEnabled() const = 0;