Create an almost-correct form_owner() for convenience
Add form submission via input element
Add click handler for <input type=submit>
Filter out buttons which aren't the submitter while constructing the dataset
Update wpt expectations
Address review comments
auto merge of #3652 : Manishearth/servo/form-button, r=jdm

Partially fixes #3647
script: Use atom comparison in more places, especially for attributes.

75% improvement in style recalc for Guardians of the Galaxy.