Add an infobar if a session is being controlled by an automated test.

This infobar is only displayed if the browser is launched with the
--enable-automation switch. It also disables the developer mode extensions
warning bubble.

Design doc:

TEST=launch with and without --enable-automation, and check for presence of automation infobar

Cr-Commit-Position: refs/heads/master@{#450257}
11 files changed