Add API for embedders to ordain their own Custom Element tag names.

The idea is to reimplement Chrome Apps' WEBVIEW element using Custom
Elements machinery for enhanced programmability through Custom
Elements' simpler "lifecycle callback" model. Later, other kinds of
extension elements will use this mechanism too.

BUG=178663,268679

Review URL: https://chromiumcodereview.appspot.com/22344002

git-svn-id: svn://svn.chromium.org/blink/trunk@155602 bbb929c8-8fbe-4397-9dbb-9b2b20218538