The WebIDL spec expects iterator objects from interfaces that declare pair
iterators to ultimately inherit from %IteratorPrototype%. Expose the
intrinsic object in the public API so we can use it in Blink's bindings
code.

BUG=chromium:689576
R=caitp@igalia.com,jkummerow@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2784543004
Cr-Commit-Position: refs/heads/master@{#44472}