[tracing] Send unique tracing process id for cross-process memory dumps identification.

base/trace_event/memory_allocator_dump_guid.h

 // Copyright 2015 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_TRACE_EVENT_MEMORY_ALLOCATOR_DUMP_GUID_H_
 #define BASE_TRACE_EVENT_MEMORY_ALLOCATOR_DUMP_GUID_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
 
 namespace base {
 namespace trace_event {
 
 class BASE_EXPORT MemoryAllocatorDumpGuid {
  public:
   MemoryAllocatorDumpGuid();
   explicit MemoryAllocatorDumpGuid(uint64 guid);
 
   // Utility ctor to hash a GUID if the caller prefers a string. The caller
-  // still has to ensure that |guid_str| is unique, per snapshot, within the
-  // global scope of all the traced processes.
+  // can insert "$$" in the string and it will be replaced by the unique child
+  // process id for creating cross-process unique guid. Otherwise the caller
+  // has to ensure that |guid_str| is unique, per snapshot, within the global
+  // scope of all the traced processes.
   explicit MemoryAllocatorDumpGuid(const std::string& guid_str);
 
+  // Sets the unique id of the child process for creating cross process unique
+  // guids.
+  static void SetUniqueChildProcessId(int child_process_id);
+
   // Returns a (hex-encoded) string representation of the guid.
   std::string ToString() const;
 
   bool empty() const { return guid_ == 0u; }
 
   bool operator==(const MemoryAllocatorDumpGuid& other) const {
     return guid_ == other.guid_;
   }
 
   bool operator!=(const MemoryAllocatorDumpGuid& other) const {
     return !(*this == other);
   }
 
  private:
   uint64 guid_;
 
   // Deliberately copy-able.
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_MEMORY_ALLOCATOR_DUMP_GUID_H_