Don't explicitly enable embedder custom elements.

They're enabled by default.

BUG=274833

Review URL: https://codereview.chromium.org/150133004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250097 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250097
Android: follow up on crrev.com/250035, remove unused import.

BUG=142642
NOTRY=True

Review URL: https://codereview.chromium.org/158963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250098 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250098