Avoid hash table lookup for every attribute

We should avoid looking at the hash table of event handler attributes
for all attributes which don't start with "on", which is most attributes.

Review URL: https://codereview.chromium.org/228943008

git-svn-id: svn://svn.chromium.org/blink/trunk@171427 bbb929c8-8fbe-4397-9dbb-9b2b20218538