viz: Add a demo app.

Add a demo app to demonstrate the usage of the viz API to write clients.
The client has no dependency on cc, or ui/compositor, and directly uses
only the viz API. The demo shows how to:
 . set up the host (for software compositing).
 . set up the service (for software compositing).
 . create connections and submit content from clients.
 . embedding an external client from an embedding client.

As mentioned, the demo only sets things up for software compositing,
which is simpler since it does not need the gpu/gl-service, and only
requires setting up the display-compositor. Future work here would be to
add a host/service combination that shows how to set up the host and
service for the gpu/gl service too.

Bug: none

Change-Id: Ie539251adba1a65e5d5dc1d2286bc7ee58ecdb49
Reviewed-on: https://chromium-review.googlesource.com/c/1394366
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627321}

diff --git a/BUILD.gn b/BUILD.gn
index 1c2b64e..281dccc8a 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -180,6 +180,7 @@
     deps += [
       "//components/cronet:cronet_tests",
       "//components/cronet:cronet_unittests",
+      "//components/viz/demo:viz_demo",
     ]
   }