Navigation transitions: added default transition stylesheet that applies in navigation transitions.

This adds a new user agent stylesheet which specifies default properties that apply to navigation transitions.

Navigation Transitions:

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/273193005

git-svn-id: svn://svn.chromium.org/blink/trunk@174637 bbb929c8-8fbe-4397-9dbb-9b2b20218538