Factored VariablesIterator out of MutableStylePropertySet.

This is in preparation for another CL that will make an inline variant that proxies through Element.

BUG=326664

Review URL: https://codereview.chromium.org/117553005

git-svn-id: svn://svn.chromium.org/blink/trunk@164197 bbb929c8-8fbe-4397-9dbb-9b2b20218538