Replace uses of hash_map in //base

base/files/file_path.h

Index: base/files/file_path.h
diff --git a/base/files/file_path.h b/base/files/file_path.h
index 0be0ad0b1049e6809574ec5dcd372462cd106d41..56dfc216ad93ab0e01dd459cf0a8be5c19769bc6 100644
--- a/base/files/file_path.h
+++ b/base/files/file_path.h
@@ -472,6 +472,8 @@ void PrintTo(const FilePath& path, std::ostream* out);
 
 // 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<>
@@ -483,4 +485,17 @@ struct hash<base::FilePath> {
 
 }  // 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_