GitLab

Thought it would be nice to keep this feature behind the experimental flag, but be able to turn it on via a finch trial from chrome.

Will be followed by: https://codereview.chromium.org/389583004/

Design doc: https://docs.google.com/a/chromium.org/document/d/17jg1RRL3RI969cLwbKBIcoGDsPwqaEdBxafGNYGwiY4/edit# 
Implementation details: https://docs.google.com/a/chromium.org/document/d/1kREPtFJaeLoDKwrfmrYTD7DHCdxX1RzFBga2gNY8lyE/edit#heading=h.bng2kpmyvxq5
BUG=370696

Review URL: https://codereview.chromium.org/386943009

git-svn-id: svn://svn.chromium.org/blink/trunk@178575 bbb929c8-8fbe-4397-9dbb-9b2b20218538