Implement custom element whitelist
          
As per an internal discussion, we would like a mechanism to enable custom
elements only for a select set of tag names. This CL implements this whitelist
functionality instead of adding an additional set of tag names.

BUG=178663

Review URL: https://chromiumcodereview.appspot.com/22652003

git-svn-id: svn://svn.chromium.org/blink/trunk@155794 bbb929c8-8fbe-4397-9dbb-9b2b20218538