Add missing interface methods.

DesktopCapturer includes a few methods that are not pure virtual because
they were added after implementions existed in Chromium. The intent was
to implement them in Chromium and then make them pure virtual, but that
never happened, which caused a bug when DesktopAndCursorComposer did not
delegate source-selection methods to the underlying capturer.

This CL adds the missing methods to a couple of simple pass-through
capturers; I will follow up with the necessary implementations for other
capturers once I've fixed the underlying remoting bug.

Change-Id: Icb3914a3cb3116878f57a9f685163c7670c1f89b
Bug: webrtc:11370
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168780
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#30550}
diff --git a/modules/desktop_capture/desktop_and_cursor_composer.h b/modules/desktop_capture/desktop_and_cursor_composer.h
index 4219c4d..8f95721 100644
--- a/modules/desktop_capture/desktop_and_cursor_composer.h
+++ b/modules/desktop_capture/desktop_and_cursor_composer.h
@@ -53,6 +53,10 @@
       std::unique_ptr<SharedMemoryFactory> shared_memory_factory) override;
   void CaptureFrame() override;
   void SetExcludedWindow(WindowId window) override;
+  bool GetSourceList(SourceList* sources) override;
+  bool SelectSource(SourceId id) override;
+  bool FocusOnSelectedSource() override;
+  bool IsOccluded(const DesktopVector& pos) override;
 
   // MouseCursorMonitor::Callback interface.
   void OnMouseCursor(MouseCursor* cursor) override;