mus: Use a barebone mus client-lib in chrome-renderer.

Notable changes:
. Add RendererWindowTreeClient that implements ui.mojom.WindowTreeClient
  interface for chrome-renderer. It currently supports the most basic
  functionalities. For example, it does not receive events (the events
  instead are routed through chrome-browser), it does not support more
  than one window, window state changes (e.g. bounds, visibility etc.)
  are ignored (the corresponding messages from chrome-browser are used
  instead) etc. Over time however, this implementation will grow.
. When chrome-renderer is running as a mus-client, it immediately creates
  a RendererWindowTreeClient instance for a RenderWidget, instead of
  waiting for a creation request from chrome-browser.
. Remove RenderWidgetMusConnection, since it's no longer needed.


Cr-Commit-Position: refs/heads/master@{#446257}
8 files changed