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

base/trace_event/memory_allocator_dump_guid.h

Index: base/trace_event/memory_allocator_dump_guid.h
diff --git a/base/trace_event/memory_allocator_dump_guid.h b/base/trace_event/memory_allocator_dump_guid.h
index 84c12ef037401fe035ce4a3f5966cd4e9f617a46..141c9464fce5124c58cd20348847a1a99e292754 100644
--- a/base/trace_event/memory_allocator_dump_guid.h
+++ b/base/trace_event/memory_allocator_dump_guid.h
@@ -19,10 +19,18 @@ class BASE_EXPORT 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 "$$" in the string and it will be replaced by the unique child
+  // process id for creating cross-process unique guid. Otherwise the caller
+  // has to ensure that |guid_str| is unique, per snapshot, within the global
+  // scope of all the traced processes.
+  // NOTE: In single process mode the "$$" does not get replaced and the caller
+  // has to set the id.
   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;