Revert "Soft slide&fade-in transition for Dialog and Popover."

This reverts r155427

BUG=274458
TBR=pfeldman

Review URL: https://codereview.chromium.org/22926007

git-svn-id: svn://svn.chromium.org/blink/trunk@156397 bbb929c8-8fbe-4397-9dbb-9b2b20218538