Simplify HeapSnapshotFilteredOrderedIterator implementation

Introduced filtered iterator and iterator over array of indexes. This allowed to decompose HeapSnapshotFilteredOrderedIterator.

BUG=None
R=alph@chromium.org

Review URL: https://codereview.chromium.org/205993003

git-svn-id: svn://svn.chromium.org/blink/trunk@169650 bbb929c8-8fbe-4397-9dbb-9b2b20218538