headless: Add a small C++ example application

Headless Shell has grown to be a little too complex to serve as an
introduction to the headless C++ API. This patch adds a simpler example
which is hopefully easier to follow. The application navigates
to a URL given on the command line, waits for it to load and prints the
DOM.

BUG=546953
R=altimin@chromium.org

Review-Url: https://codereview.chromium.org/2677033003 .
Cr-Commit-Position: refs/heads/master@{#450025}
5 files changed