Add support for shared texture and external BeginFrame in OSR mode (issue #1006)

- Add CefWindowInfo::shared_texture_enabled and
  CefRenderHandler::OnAcceleratedPaint for shared texture support. Currently
  only supported on Windows (D3D11).
- Add CefWindowInfo::external_begin_frame_enabled and
  CefBrowserHost::SendExternalBeginFrame for external begin frame support.

libcef/browser/browser_platform_delegate.cc

@@ -121,6 +121,10 @@ void CefBrowserPlatformDelegate::Invalidate(cef_paint_element_type_t type) {
   NOTREACHED();
 }
 
+void CefBrowserPlatformDelegate::SendExternalBeginFrame() {
+  NOTREACHED();
+}
+
 void CefBrowserPlatformDelegate::SetWindowlessFrameRate(int frame_rate) {
   NOTREACHED();
 }