Don't come undone when undoing the editing of a Custom Element.

CustomElementRegistrationContext hitherto asserted that the parser
would only call setAttribute for the "is" attribute once (or for any
given attribute; but the "is" attribute has special significance
because it governs Custom Element type extensions).

However undoing ReplaceNodeWithSpan may pull an existing Custom
Element out and "parser set" the "is" attribute a second time. This
change weakens the assertion and simply makes subsequent sets of "is"
a no-op.

This defect was found by the fuzzing described in
<http://crbug.com/234509#c40>.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/22320009

git-svn-id: svn://svn.chromium.org/blink/trunk@155612 bbb929c8-8fbe-4397-9dbb-9b2b20218538