GitLab

This CL fixes a bug when hiding and showing transition elements and adds the test for it.

BUG=370696

Review URL: https://codereview.chromium.org/723073003

git-svn-id: svn://svn.chromium.org/blink/trunk@185328 bbb929c8-8fbe-4397-9dbb-9b2b20218538