Simplify CustomElementScheduler by passing the enum for callback type directly.

Instead of having one scheduling function per callback type, we can use
a single shared one except for the attribute callback case.

Review URL: https://codereview.chromium.org/220013003

git-svn-id: svn://svn.chromium.org/blink/trunk@170500 bbb929c8-8fbe-4397-9dbb-9b2b20218538