trace_processor: group all kernel threads under kthreadd

This change forces all kernel threads (i.e. threads with parent process
2 - the kthreadd process) to be grouped under kthreadd as threads
instead of as new processes.

Change-Id: I4e84e56f3a0ff35f295b759d54c1493bc5522dc3
Bug: 136635390
diff --git a/src/trace_processor/proto_trace_parser_unittest.cc b/src/trace_processor/proto_trace_parser_unittest.cc
index 5027c38..b3a2b9f 100644
--- a/src/trace_processor/proto_trace_parser_unittest.cc
+++ b/src/trace_processor/proto_trace_parser_unittest.cc
@@ -527,10 +527,10 @@
 
   process->add_cmdline(kProcName1);
   process->set_pid(1);
-  process->set_ppid(2);
+  process->set_ppid(3);
 
   EXPECT_CALL(*process_,
-              SetProcessMetadata(1, Eq(2u), base::StringView(kProcName1)));
+              SetProcessMetadata(1, Eq(3u), base::StringView(kProcName1)));
   Tokenize();
 }
 
@@ -543,10 +543,10 @@
   process->add_cmdline(kProcName1);
   process->add_cmdline(kProcName2);
   process->set_pid(1);
-  process->set_ppid(2);
+  process->set_ppid(3);
 
   EXPECT_CALL(*process_,
-              SetProcessMetadata(1, Eq(2u), base::StringView(kProcName1)));
+              SetProcessMetadata(1, Eq(3u), base::StringView(kProcName1)));
   Tokenize();
 }