Move handling of 'attributeType' to SVGAnimateElement

Another animation property that applies only to the SVGAnimateElement
part/subtree of the element hierarchy. Move it down for continued
"unlocking" of the structure.

BUG=641437

Review-Url: https://codereview.chromium.org/2391993006
Cr-Commit-Position: refs/heads/master@{#423467}
4 files changed