Replace uses of hash_map in //base

base/files/file_path.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // FilePath is a container for pathnames stored in a platform's native string
 // type, providing containers for manipulation in according with the
 // platform's conventions for pathnames.  It supports the following path
 // types:
 //
 //                   POSIX            Windows
@@ skipping 454 matching lines @@
 #if defined(OS_POSIX)
 #define FILE_PATH_LITERAL(x) x
 #define PRFilePath "s"
 #elif defined(OS_WIN)
 #define FILE_PATH_LITERAL(x) L ## x
 #define PRFilePath "ls"
 #endif  // OS_WIN
 
 // Provide a hash function so that hash_sets and maps can contain FilePath
 // objects.
+//
+// TODO(brettw) remove this when base::hash_map is removed.

[Łukasz Anforowicz, 2017/04/24 18:46:10]

I don't understand why we need to wait for removal of base::hash_map.

I see that the default BASE_HASH_NAMESPACE::hash<T> (defined in
base/containers/hash_tables.h) forwards to std::hash<base::FilePath> that you
are adding below.  Doesn't this mean that we can remove the
BASE_HASH_NAMESPACE::hash<base::FilePath> specialization today (i.e. without
waiting for removal of base::hash_map)?

PS. I guess I am fine with doing the removal in a separate CL, I am just trying
to understand what is going on in this CL :-)

PPS. If we could remove references to BASE_HASH_NAMESPACE in
base/files/file_path.h, then we could also (I think) remove #include of
base/containers/hash_tables.h (and it seems desirable to limit who
[transitively] includes this file).

[brettw, 2017/04/25 23:32:36]

I missed the forwarding to std::hash. I'll remove.

Removing the include forced me to fix a smattering of random files across the
code base that were relying on this include for either base/hash.h and
base/containers/hash_tables.h

 namespace BASE_HASH_NAMESPACE {
 
 template<>
 struct hash<base::FilePath> {
   size_t operator()(const base::FilePath& f) const {
     return hash<base::FilePath::StringType>()(f.value());
   }
 };
 
 }  // namespace BASE_HASH_NAMESPACE
 
+namespace std {
+
+template <>
+struct hash<base::FilePath> {
+  typedef base::FilePath argument_type;
+  typedef std::size_t result_type;
+  result_type operator()(argument_type const& f) const {
+    return hash<base::FilePath::StringType>()(f.value());
+  }
+};
+
+}  // namespace std
+
 #endif  // BASE_FILES_FILE_PATH_H_