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: https://docs.google.com/document/d/1JYj9K61UyxIYavR8_HATYIglR9T_rDwAtLLsD3fbDQg/edit

BUG=chromedriver:1625
TEST=launch with and without --enable-automation, and check for presence of automation infobar

Review-Url: https://codereview.chromium.org/2564973002
Cr-Commit-Position: refs/heads/master@{#450257}
11 files changed