Replace base::hash_set with std::unordered_set

base::hash_set is alias of std::unordered_set now.

I applied following command to make this CL.

$ git grep -l 'base::hash_set' | fgrep -v base/ | xargs sed -i -e 's/base::hash_set/std::unordered_set/g' -e 's/\(#include <[a-z_]*>\)/\1\n#include <unordered_set>/' -e 's/BASE_HASH_NAMESPACE/std/'
$ git cl format

TBR: dcheng@chromium.org
Bug: 576864
Change-Id: I89535173626483c06989b4d8a99dc46c1d49ce7b
Reviewed-on: https://chromium-review.googlesource.com/c/1390887
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#620168}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8332bf9da1576fe59109cdc2de8a06added6873f
diff --git a/difference_estimator.cc b/difference_estimator.cc
index 099900c..82d456d 100644
--- a/difference_estimator.cc
+++ b/difference_estimator.cc
@@ -60,7 +60,7 @@
 
  private:
   Region region_;
-  base::hash_set<size_t> hashes_;
+  std::unordered_set<size_t> hashes_;
 
   friend class DifferenceEstimator;
   DISALLOW_COPY_AND_ASSIGN(Base);