Make Custom Element registration context creation explicit.

This brings the implementation into line with the spec, which
specifies when a document *does* get a new registration context, or
shares an existing one, instead of applying a heuristic based on
document type. See
<http://w3c.github.io/webcomponents/spec/custom/#creating-and-passing-registries>

BUG=339431

Review URL: https://codereview.chromium.org/132203012

git-svn-id: svn://svn.chromium.org/blink/trunk@166359 bbb929c8-8fbe-4397-9dbb-9b2b20218538