Allow std::unordered_*.

This is a reland of https://codereview.chromium.org/1502373009 with some fixes
for components/metrics/leak_detector allocator type mismatches.

Original issue's description:
> Allow std::unordered_*.
>
> base::hash_* is, as a transition step, implemented in terms of
> std::unordered_*. Later commits will convert existing uses.
>
> Also fix a host of IWYU problems that arose from this CL.
>
> (NOPRESUBMIT because the wstring presubmit check is overzealous
> and complains about the reference to wstring in the comment.)
>
> Committed: https://crrev.com/3f37f7f1459e7b5a452c0e433493e0a6e9649ca7
> Cr-Commit-Position: refs/heads/master@{#370553}

BUG=576864
TBR=derat@chromium.org,danakj@chromium.org,dalecurtis@chromium.org,jbauman@chromium.org,blundell@chromium.org
NOPRESUBMIT=true
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1615713003

Cr-Commit-Position: refs/heads/master@{#370867}

3 files changed, 31 insertions, 6 deletions

diff --git a/base/strings/string16.h b/base/strings/string16.h
index e47669c..82dd0fa 100644
--- a/base/strings/string16.h
+++ b/base/strings/string16.h
@@ -29,6 +29,8 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+
+#include <functional>
 #include <string>
 
 #include "base/base_export.h"
@@ -182,6 +184,21 @@ BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 extern template
 class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
 
+// Specialize std::hash for base::string16. Although the style guide forbids
+// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16
+// platforms, where base::string16 is a type alias for std::wstring.
+namespace std {
+template <>
+struct hash<base::string16> {
+  std::size_t operator()(const base::string16& s) const {
+    std::size_t result = 0;
+    for (base::char16 c : s)
+      result = (result * 131) + c;
+    return result;
+  }
+};
+}  // namespace std
+
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRINGS_STRING16_H_
diff --git a/base/strings/string16_unittest.cc b/base/strings/string16_unittest.cc
index 4e58218..0d2ca80 100644
--- a/base/strings/string16_unittest.cc
+++ b/base/strings/string16_unittest.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include <sstream>
+#include <unordered_set>
 
 #include "base/strings/string16.h"
 
@@ -11,8 +12,6 @@
 
 namespace base {
 
-#if defined(WCHAR_T_IS_UTF32)
-
 // We define a custom operator<< for string16 so we can use it with logging.
 // This tests that conversion.
 TEST(String16Test, OutputStream) {
@@ -53,6 +52,15 @@ TEST(String16Test, OutputStream) {
   }
 }
 
-#endif
+TEST(String16Test, Hash) {
+  string16 str1 = ASCIIToUTF16("hello");
+  string16 str2 = ASCIIToUTF16("world");
+
+  std::unordered_set<string16> set;
+
+  set.insert(str1);
+  EXPECT_EQ(1u, set.count(str1));
+  EXPECT_EQ(0u, set.count(str2));
+}
 
 }  // namespace base
diff --git a/base/strings/string_piece.h b/base/strings/string_piece.h
index 31e7596..92634b9 100644
--- a/base/strings/string_piece.h
+++ b/base/strings/string_piece.h
@@ -439,9 +439,9 @@ BASE_EXPORT std::ostream& operator<<(std::ostream& o,
 // We provide appropriate hash functions so StringPiece and StringPiece16 can
 // be used as keys in hash sets and maps.
 
-// This hash function is copied from base/containers/hash_tables.h. We don't
-// use the ones already defined for string and string16 directly because it
-// would require the string constructors to be called, which we don't want.
+// This hash function is copied from base/strings/string16.h. We don't use the
+// ones already defined for string and string16 directly because it would
+// require the string constructors to be called, which we don't want.
 #define HASH_STRING_PIECE(StringPieceType, string_piece)                \
   std::size_t result = 0;                                               \
   for (StringPieceType::const_iterator i = string_piece.begin();        \