Changed the mojom type of the ViewManager::Embed() ServiceProvider parameter from |ServiceProvider?| to |ServiceProvider&?|.

More details in the bug report.

In addition to running the tests I verified that the wm_flow demo still displays its HelloWorld() ack messages (and it behaves the same as it did before the change).

mojo/tools/mojo_demo.sh --wm_flow

BUG=432200
R=sky@chromium.org

Review URL: https://codereview.chromium.org/715263004