headless: Add devtools discovery page

This page adds a generated discovery page which can be used to pick a
tab to inspect using devtools. This is based on the similar page
generated by Content Shell.

BUG=546953
TEST=headless_shell --remote-debugging-port=9222  # Navigate to localhost:9222 to see the google.com tab.

Review URL: https://codereview.chromium.org/1698403004

Cr-Commit-Position: refs/heads/master@{#378759}
5 files changed