Rename the Custom Elements runtime feature to "CustomElements".

The name of the feature should match the name of the spec and the name
used in the implementation. It used to be called CustomDOMElements.

Refactoring; no change in behavior.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23189010

git-svn-id: svn://svn.chromium.org/blink/trunk@156258 bbb929c8-8fbe-4397-9dbb-9b2b20218538