Remove attributeCount() / attributeAt() API from Element

Remove attributeCount() / attributeAt() API from Element and port callers to
the new AttributeCollection API instead. This avoids API duplication between
Element and AttributeCollection. This also usually makes the calling code
more efficient as it avoids redundant (implicit) ElementData::isUnique()
checks and potentially leverages iterators.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/357033002

git-svn-id: svn://svn.chromium.org/blink/trunk@178056 bbb929c8-8fbe-4397-9dbb-9b2b20218538