Make CollectionIndexCache::setCachedNode() private

Make CollectionIndexCache::setCachedNode() private. We no longer need it to be
public as it is never called from the outside. It is better for the class to
handle its own cache.

R=rob.buis@samsung.com

Review URL: https://codereview.chromium.org/167333003

git-svn-id: svn://svn.chromium.org/blink/trunk@167222 bbb929c8-8fbe-4397-9dbb-9b2b20218538