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}
  base::FilePath metrics_file() {
    return temp_dir_.GetPath().AppendASCII(kMetricsFilename);
  }

  base::FilePath metrics_file() {
    return temp_dir_.GetPath().AppendASCII(kMetricsFilename);
  }
