Adds CaptureClientObserver as a way to track changes in capture

ui/views/widget/desktop_aura/desktop_capture_client.cc

Index: ui/views/widget/desktop_aura/desktop_capture_client.cc
diff --git a/ui/views/widget/desktop_aura/desktop_capture_client.cc b/ui/views/widget/desktop_aura/desktop_capture_client.cc
index f2f3cf1cf056cea175f4668532de9d7ac2b58c6c..5ce64e9d025154dfaedc259308675d4446709e43 100644
--- a/ui/views/widget/desktop_aura/desktop_capture_client.cc
+++ b/ui/views/widget/desktop_aura/desktop_capture_client.cc
@@ -98,4 +98,14 @@ aura::Window* DesktopCaptureClient::GetGlobalCaptureWindow() {
   return NULL;
 }
 
+void DesktopCaptureClient::AddObserver(
+    aura::client::CaptureClientObserver* observer) {
+  NOTREACHED();
+}
+
+void DesktopCaptureClient::RemoveObserver(
+    aura::client::CaptureClientObserver* observer) {
+  NOTREACHED();
+}
+
 }  // namespace views