Would you want to just auto-allow everything in headless mode, or something else?

I think we are going to "auto-ignore" everything in headless (pretend the
user just closes the permission prompt without accepting or denying).
According to the headless team in London this makes more sense, plus it
happens to be the desired behaviour to make one of the Lighthouse checks
work. Specifically, the check that a website doesnt request a Notification
or Geolocation permission in onload.
-Oleg

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8bdb9db76cebc9398316e15e2669837f99bea61c

commit 8bdb9db76cebc9398316e15e2669837f99bea61c
Author: sushkov <sushkov@chromium.org>
Date: Tue Mar 28 07:29:43 2017

Added headless permission manager. This permissions manager simply acks all permission requests without accepting or denying.

BUG= 704820 

Review-Url: https://codereview.chromium.org/2771993003
Cr-Commit-Position: refs/heads/master@{#460036}

[modify] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/BUILD.gn
[modify] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/lib/browser/headless_browser_context_impl.cc
[modify] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/lib/browser/headless_browser_context_impl.h
[add] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/lib/browser/headless_permission_manager.cc
[add] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/lib/browser/headless_permission_manager.h
[modify] https://crrev.com/8bdb9db76cebc9398316e15e2669837f99bea61c/headless/lib/headless_browser_browsertest.cc