Make "embedder custom elements" on by default.

This isn't a web-visible feature. It's used by C++ embedders to
provide bespoke elements, even if Custom Elements aren't
enabled. Since Custom Elements are enabled by default, it makes sense
to simply enable embedder elements by default too. Embedder elements
don't do much unless you register an element anyway.

BUG=274833

Review URL: https://codereview.chromium.org/153863004

git-svn-id: svn://svn.chromium.org/blink/trunk@166787 bbb929c8-8fbe-4397-9dbb-9b2b20218538