Remove the Custom Element registration context type extension map.

A given custom element either has a definition associated with it, or
goes into the upgrade candidate map. Both of these places encode
whether the custom element is a type extension or not. Thus there is
no need for a side table recording the type extension of a given
element.

BUG=255725
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19709003

git-svn-id: svn://svn.chromium.org/blink/trunk@154869 bbb929c8-8fbe-4397-9dbb-9b2b20218538