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

components/tracing/tracing_messages.h

Index: components/tracing/tracing_messages.h
diff --git a/components/tracing/tracing_messages.h b/components/tracing/tracing_messages.h
index 14f31bb1079fb8ee27d62d86f2422b83f2e738ed..6851d9089bac36eef96a48a1f95816ac3a37e4e0 100644
--- a/components/tracing/tracing_messages.h
+++ b/components/tracing/tracing_messages.h
@@ -32,9 +32,10 @@ IPC_ENUM_TRAITS_MAX_VALUE(
     static_cast<int>(base::trace_event::MemoryDumpType::LAST))
 
 // Sent to all child processes to enable trace event recording.
-IPC_MESSAGE_CONTROL2(TracingMsg_BeginTracing,
+IPC_MESSAGE_CONTROL3(TracingMsg_BeginTracing,
                      std::string /*  trace_config_str */,
-                     base::TraceTicks /* browser_time */)
+                     base::TraceTicks /* browser_time */,
+                     int /* Unique child process id */)
 
 // Sent to all child processes to disable trace event recording.
 IPC_MESSAGE_CONTROL0(TracingMsg_EndTracing)