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

src/base/thread_task_runner_unittest.cc

diff --git a/src/base/thread_task_runner_unittest.cc b/src/base/thread_task_runner_unittest.cc
index c4356fe..0e7f2af 100644
--- a/src/base/thread_task_runner_unittest.cc
+++ b/src/base/thread_task_runner_unittest.cc
@@ -18,8 +18,8 @@
 
 #include <thread>
 
-#include <gtest/gtest.h>
 #include "perfetto/ext/base/thread_checker.h"
+#include "test/gtest_and_gmock.h"
 
 namespace perfetto {
 namespace base {