Revert "Ensure elementData is valid in classAttributeChanged."

This reverts commit r156043 due to a crash in SpaceSplitString::set.

BUG=272954

Review URL: https://chromiumcodereview.appspot.com/23066006

git-svn-id: svn://svn.chromium.org/blink/trunk@156111 bbb929c8-8fbe-4397-9dbb-9b2b20218538