Clear mutable inline style when it is empty.

As part of this, add a new InlineVariablesIterator class. This implements a
CSSVariablesIterator interface, but in a way that is compatible with how inline
styles as represented in a MutableStylePropertySet are accessed through
ElementRareData rather than a ref pointer. This allows us to decouple the
lifetimes of a CSSVariablesIterator and the MutableStylePropertySet that it is a
helper/iterator for.

BUG=326664

Review URL: https://codereview.chromium.org/119533003

git-svn-id: svn://svn.chromium.org/blink/trunk@164307 bbb929c8-8fbe-4397-9dbb-9b2b20218538