Remove the Custom Element registration context null object.

core calls the Custom Element registration context at fewer points
now, so the null registration context isn't really pulling its weight.

BUG=247219
R=morrita@chromium.org

Review URL: https://codereview.chromium.org/19810005

git-svn-id: svn://svn.chromium.org/blink/trunk@154878 bbb929c8-8fbe-4397-9dbb-9b2b20218538