Adds CaptureClientObserver as a way to track changes in capture

I'm going to need this for the aura-mus work so that what is the
WindowTreeClient implementation can track capture and update the
server appropriately.

I didn't update the desktop client as I don't think I'll need that one.

BUG=none
TEST=none
R=sadrul@chromium.org

Review-Url: https://chromiumcodereview.appspot.com/2436023003
Cr-Commit-Position: refs/heads/master@{#426848}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 2fda230..457615b5 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -34,6 +34,7 @@
     "client/aura_constants.h",
     "client/capture_client.cc",
     "client/capture_client.h",
+    "client/capture_client_observer.h",
     "client/capture_delegate.h",
     "client/cursor_client.cc",
     "client/cursor_client.h",