Makefile: Fixed paths vulcanized build.html.

Previously, was generating relative paths such as "../../app/style.css"
(which are invalid in the built app). Now, generates absolute paths
(relative to the app directory), such as "/style.css". These paths are
valid within the running app.

Updated background.js to launch build.html instead of main.html.

R=hbeder@google.com

Review URL: https://codereview.appspot.com/182230043