Update from https://crrev.com/320931

base/location.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.
 
 #ifndef BASE_LOCATION_H_
 #define BASE_LOCATION_H_
 
+#include <cassert>
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/containers/hash_tables.h"
 
 namespace tracked_objects {
 
 // Location provides basic info where of an object was constructed, or was
 // significantly brought to life.
 class BASE_EXPORT Location {
  public:
   // Constructor should be called with a long-lived char*, such as __FILE__.
   // It assumes the provided value will persist as a global constant, and it
   // will not make a copy of it.
   Location(const char* function_name,
            const char* file_name,
            int line_number,
            const void* program_counter);
 
   // Provide a default constructor for easy of debugging.
   Location();
 
-  // Comparison operator for insertion into a std::map<> hash tables.
-  // All we need is *some* (any) hashing distinction.  Strings should already
-  // be unique, so we don't bother with strcmp or such.
-  // Use line number as the primary key (because it is fast, and usually gets us
-  // a difference), and then pointers as secondary keys (just to get some
-  // distinctions).
-  bool operator < (const Location& other) const {
-    if (line_number_ != other.line_number_)
-      return line_number_ < other.line_number_;
-    if (file_name_ != other.file_name_)
-      return file_name_ < other.file_name_;
-    return function_name_ < other.function_name_;
+  // Copy constructor.
+  Location(const Location& other);
+
+  // Comparator for hash map insertion.
+  // No need to use |function_name_| since the other two fields uniquely
+  // identify this location.
+  bool operator==(const Location& other) const {
+    return line_number_ == other.line_number_ &&
+           file_name_ == other.file_name_;
   }
 
   const char* function_name()   const { return function_name_; }
   const char* file_name()       const { return file_name_; }
   int line_number()             const { return line_number_; }
   const void* program_counter() const { return program_counter_; }
 
   std::string ToString() const;
 
+  // Hash operator for hash maps.
+  struct Hash {
+    size_t operator()(const Location& location) const {
+      // Compute the hash value using file name pointer and line number.
+      // No need to use |function_name_| since the other two fields uniquely
+      // identify this location.
+
+      // The file name will always be uniquely identified by its pointer since
+      // it comes from __FILE__, so no need to check the contents of the string.
+      // See the definition of FROM_HERE in location.h, and how it is used
+      // elsewhere.
+
+      // Due to inconsistent definitions of uint64_t and uintptr_t, casting the
+      // file name pointer to a uintptr_t causes a compiler error for some
+      // platforms. The solution is to explicitly cast it to a uint64_t.
+      return base::HashPair(reinterpret_cast<uint64_t>(location.file_name()),
+                            location.line_number());
+    }
+  };
+
   // Translate the some of the state in this instance into a human readable
   // string with HTML characters in the function names escaped, and append that
   // string to |output|.  Inclusion of the file_name_ and function_name_ are
   // optional, and controlled by the boolean arguments.
   void Write(bool display_filename, bool display_function_name,
              std::string* output) const;
 
   // Write function_name_ in HTML with '<' and '>' properly encoded.
   void WriteFunctionName(std::string* output) const;
 
@@ skipping 24 matching lines @@
 
 #define FROM_HERE_WITH_EXPLICIT_FUNCTION(function_name)                        \
     ::tracked_objects::Location(function_name,                                 \
                                 __FILE__,                                      \
                                 __LINE__,                                      \
                                 ::tracked_objects::GetProgramCounter())
 
 }  // namespace tracked_objects
 
 #endif  // BASE_LOCATION_H_