Make CustomElementSchedular static

This patch removes the instance() function and adds a new function
callbackQueues() to keep the map in the schedular itself. This
function will return the map.

Review URL: https://codereview.chromium.org/528373003

git-svn-id: svn://svn.chromium.org/blink/trunk@181456 bbb929c8-8fbe-4397-9dbb-9b2b20218538