Navigation transitions: Added createStyledMarkupForNavigationTransition

This will be used to serialize markup for transition elements, which is then sent to the transition layer.

BUG=370696

Review URL: https://codereview.chromium.org/281383006

git-svn-id: svn://svn.chromium.org/blink/trunk@178403 bbb929c8-8fbe-4397-9dbb-9b2b20218538