Update Custom Elements API to new names.

document.register => document.registerElement
enteredViewCallback => attachedCallback
leftViewCallback => detachedCallback

This also removes the long-deprecated webkitRegister method.

BUG=180965
R=haraken@chromium.org, morrita@chromium.org

Review URL: https://codereview.chromium.org/117313008

git-svn-id: svn://svn.chromium.org/blink/trunk@164213 bbb929c8-8fbe-4397-9dbb-9b2b20218538