OrderedHashSet doesn't need a ::Get, so we can move it to
OrderedHashMap.

Bug: v8:5717
Change-Id: I9606d8c4608473f9daecf8a87b4dd2e3b9570246
Reviewed-on: https://chromium-review.googlesource.com/522348Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45740}