Push attributeName handling down into SVGAnimateElement

SVGAnimateElement (and derived classes) is where 'attributeName' has any
meaning. Move setting/updating of the resolved 'attributeName' value
there. This allows setAttributeName to be devirtualized.
Storage is kept at the SVGSMILElement level because of how it's used at
schedule/unschedule time.

BUG=641437

Review-Url: https://codereview.chromium.org/2602423002
Cr-Commit-Position: refs/heads/master@{#441356}
4 files changed