This CL causes sky/shell to create a blink::WebView to show that sky/shell
links with sky/engine. In the process, I've made it easier to be a trivial
embedder of sky/engine by removing the requirement to implement
blink::ServiceProvider.

This CL also causes sky/shell to link with mojo/edk/system to resolve link
errors with Mojo fabric (e.g., MojoClose, MojoWriteMessage, etc). To make this
work properly, we'll need to initialize the EDK in a future CL.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/873923003