Remove HeapSnapshotItemIterator.rewind

We don't actually need to rewind iterators. The iterators should be one-pass only and rewind just complicates the code.

BUG=None

Review URL: https://codereview.chromium.org/206393004

git-svn-id: svn://svn.chromium.org/blink/trunk@169654 bbb929c8-8fbe-4397-9dbb-9b2b20218538