Remove UMA.CreatePersistentHistogram.Result

This histogram isn't showing anything meaningful and the problems it
could show are better observed by looking at the allocators directly.

Bug: 831013
Change-Id: Ibe968597758230192e53a7675e7390e968c9e5b9
Reviewed-on: https://chromium-review.googlesource.com/1008047
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549986}
PersistentHistogramAllocator::Iterator::GetNextWithIgnore(Reference ignore) {
  PersistentMemoryAllocator::Reference ref;
  while ((ref = memory_iter_.GetNextOfType<PersistentHistogramData>()) != 0) {
    if (ref != ignore)
      return allocator_->GetHistogram(ref);
  }
  return nullptr;
}

PersistentHistogramAllocator::Iterator::GetNextWithIgnore(Reference ignore) {
  PersistentMemoryAllocator::Reference ref;
  while ((ref = memory_iter_.GetNextOfType<PersistentHistogramData>()) != 0) {
    if (ref != ignore)
      return allocator_->GetHistogram(ref);
  }
  return nullptr;
}
