Initial work on a new <view> element backed by a mojo::View.

This CL introduces an HTMLViewElement which, when inserted into a document,
causes a mojo::View to be created and navigated to the provided URL. No
compositing is done, but the view manager handles the rendering (as I
understand it).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/708903002