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.

libcef/browser/osr/browser_platform_delegate_osr.cc

@@ -8,6 +8,7 @@
 
 #include "libcef/browser/browser_host_impl.h"
 #include "libcef/browser/image_impl.h"
+#include "libcef/browser/osr/osr_accessibility_util.h"
 #include "libcef/browser/osr/render_widget_host_view_osr.h"
 #include "libcef/browser/osr/web_contents_view_osr.h"
 #include "libcef/common/drag_data_impl.h"
@@ -522,6 +523,34 @@ void CefBrowserPlatformDelegateOsr::DragSourceSystemDragEnded() {
   drag_start_rwh_ = nullptr;
 }
 
+void CefBrowserPlatformDelegateOsr::AccessibilityEventReceived(
+    const std::vector<content::AXEventNotificationDetails>& eventData) {
+  CefRefPtr<CefRenderHandler> handler = browser_->client()->GetRenderHandler();
+  if (handler.get()) {
+    CefRefPtr<CefAccessibilityHandler> acchandler =
+        handler->GetAccessibilityHandler();
+
+    if (acchandler.get()) {
+      acchandler->OnAccessibilityTreeChange(
+          osr_accessibility_util::ParseAccessibilityEventData(eventData));
+    }
+  }
+}
+
+void CefBrowserPlatformDelegateOsr::AccessibilityLocationChangesReceived(
+    const std::vector<content::AXLocationChangeNotificationDetails>& locData) {
+  CefRefPtr<CefRenderHandler> handler = browser_->client()->GetRenderHandler();
+  if (handler.get()) {
+    CefRefPtr<CefAccessibilityHandler> acchandler =
+        handler->GetAccessibilityHandler();
+
+    if (acchandler.get()) {
+      acchandler->OnAccessibilityLocationChange(
+          osr_accessibility_util::ParseAccessibilityLocationData(locData));
+    }
+  }
+}
+
 CefWindowHandle CefBrowserPlatformDelegateOsr::GetParentWindowHandle() const {
   return GetHostWindowHandle();
 }