Implement accessibility enhancements (issue #1217)

- Add new CefBrowserHost::SetAccessibilityState method for toggling
  accessibility state when readers are detected by the client.
- Add new CefAccessibilityHandler interface for the delivery of
  accessibility notifications to windowless (OSR) clients.
- Fix delivery of CefFocusHandler callbacks to windowless clients.
- cefclient: Add example windowless accessibility implementation on Windows and macOS.
- cefclient: Automatically detect screen readers on Windows and macOS.

tests/cefclient/browser/client_handler_osr.h

@@ -13,6 +13,7 @@ namespace client {
 // Client handler implementation for windowless browsers. There will only ever
 // be one browser per handler instance.
 class ClientHandlerOsr : public ClientHandler,
+                         public CefAccessibilityHandler,
                          public CefRenderHandler {
  public:
   // Implement this interface to receive notification of ClientHandlerOsr
@@ -61,6 +62,8 @@ class ClientHandlerOsr : public ClientHandler,
         const CefRange& selection_range,
         const CefRenderHandler::RectList& character_bounds) = 0;
 
+    virtual void UpdateAccessibilityTree(CefRefPtr<CefValue> value) = 0;
+
    protected:
     virtual ~OsrDelegate() {}
   };
@@ -77,6 +80,9 @@ class ClientHandlerOsr : public ClientHandler,
   CefRefPtr<CefRenderHandler> GetRenderHandler() OVERRIDE {
     return this;
   }
+  CefRefPtr<CefAccessibilityHandler> GetAccessibilityHandler() OVERRIDE {
+    return this;
+  }
 
   // CefLifeSpanHandler methods.
   void OnAfterCreated(CefRefPtr<CefBrowser> browser) OVERRIDE;
@@ -118,6 +124,10 @@ class ClientHandlerOsr : public ClientHandler,
       const CefRange& selection_range,
       const CefRenderHandler::RectList& character_bounds) OVERRIDE;
 
+  // CefAccessibilityHandler methods.
+  void OnAccessibilityTreeChange(CefRefPtr<CefValue> value) OVERRIDE;
+  void OnAccessibilityLocationChange(CefRefPtr<CefValue> value) OVERRIDE {}
+
  private:
   // Only accessed on the UI thread.
   OsrDelegate* osr_delegate_;