Refactor StringPiece hash.

Rename struct BASE_HASH_NAMESPACE::hash<base::StringPiece> to struct
base::StringPieceHash, and modify existing code as necessary.  This is motivated
by https://crrev.com/1660273002/#msg6.

* Remove #include "base/containers/hash_tables.h" where not used any more.
* Add this include for hash_map where necessary (string_piece.h does not include
  it any longer, so this might be necessary for the code to compile).
* Change net::linked_hash_map from base::hash_map to std::unordered_map and add
  Hash template parameter.  This way BASE_HASH_NAMESPACE::hash<Key> will not be
  implicitly used any more, so move QuicBlockedWriterInterfacePtrHash out of
  that namespace.

BUG=576864

Review URL: https://codereview.chromium.org/1666843002

Cr-Original-Commit-Position: refs/heads/master@{#374005}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 63c013002ffbbb4c7eac172097e16ea907403e08

tools/gn/scope.h

diff --git a/tools/gn/scope.h b/tools/gn/scope.h
index 2554366..96b3939 100644
--- a/tools/gn/scope.h
+++ b/tools/gn/scope.h
@@ -40,7 +40,8 @@
 // variables. So you should use a non-const containing scope whenever possible.
 class Scope {
  public:
-  typedef base::hash_map<base::StringPiece, Value> KeyValueMap;
+  typedef base::hash_map<base::StringPiece, Value, base::StringPieceHash>
+      KeyValueMap;
   // Holds an owning list of Items.
   typedef ScopedVector<Item> ItemVector;
 
@@ -329,7 +330,8 @@
   // for more.
   unsigned mode_flags_;
 
-  typedef base::hash_map<base::StringPiece, Record> RecordMap;
+  typedef base::hash_map<base::StringPiece, Record, base::StringPieceHash>
+      RecordMap;
   RecordMap values_;
 
   // Owning pointers. Note that this can't use string pieces since the names