PMurHash comes from the smhasher repository at chromium/src/third_party/smhasher

Bug: 697758
Change-Id: Id2859edf37ae66bf27509d53db7f22db8831fe44
Reviewed-on: https://chromium-review.googlesource.com/687970
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>