Drop findAttribute*ByName() API from Element

Drop findAttribute*ByName() API from Element and use the AttributeCollection
API instead. This makes it more likely for callers to reuse to
AttributeCollection and thus avoid unnecessary and implicit
ElementData::isUnique() checks. This also avoid API duplication between Element
and AttributeCollection.

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/387413003

git-svn-id: svn://svn.chromium.org/blink/trunk@178095 bbb929c8-8fbe-4397-9dbb-9b2b20218538