Navigation transitions (web to native app): Hide/Show transition elements (Blink side)

We need a way to show hidden transition elements when coming back from native app (when back button is pressed).

This is the Blink side of the CL. The Chrome side is here: https://codereview.chromium.org/712183002/

BUG=370696

Review URL: https://codereview.chromium.org/715653004

git-svn-id: svn://svn.chromium.org/blink/trunk@185162 bbb929c8-8fbe-4397-9dbb-9b2b20218538