Remove use of -isystem for googletest

Use standard include_dirs and a proxy header to manually suppress
warning. This solves the problem of sign conversion warning that
fire only when rolling into chrome.
Follow-up CLs will try to remove -isystem also to other
buildtools. Limiting the scope of this for now.

Change-Id: I8bc7924a95c7dd73cad54f447c122bd339e4e2a4
Bug: 129130827
diff --git a/src/tracing/core/tracing_service_impl_unittest.cc b/src/tracing/core/tracing_service_impl_unittest.cc
index 7a26c2a..beed0fc 100644
--- a/src/tracing/core/tracing_service_impl_unittest.cc
+++ b/src/tracing/core/tracing_service_impl_unittest.cc
@@ -18,8 +18,6 @@
 
 #include <string.h>
 
-#include <gmock/gmock.h>
-#include <gtest/gtest.h>
 #include "perfetto/ext/base/file_utils.h"
 #include "perfetto/ext/base/temp_file.h"
 #include "perfetto/ext/base/utils.h"
@@ -36,6 +34,7 @@
 #include "src/tracing/test/mock_consumer.h"
 #include "src/tracing/test/mock_producer.h"
 #include "src/tracing/test/test_shared_memory.h"
+#include "test/gtest_and_gmock.h"
 
 #include "perfetto/trace/test_event.pbzero.h"
 #include "perfetto/trace/trace.pb.h"