Revert 165068 "Ensure ElementData exists before modifying when a..."

Caused UAFs on ClusterFuzz.

> Ensure ElementData exists before modifying when animating class attribute
> 
> This is a merge of http://trac.webkit.org/changeset/153835
> by Rob Buis <rwlbuis@webkit.org>.
> 
> BUG=334194
> 
> Review URL: https://codereview.chromium.org/138173002

TBR=fs@opera.com

Review URL: https://codereview.chromium.org/130843006

git-svn-id: svn://svn.chromium.org/blink/trunk@165200 bbb929c8-8fbe-4397-9dbb-9b2b20218538