Remove unnecessary call to the deprecated g_thread_init().

g_thread_init() has been deprecated since glib 2.32. This CL removes the
unnecessary call to g_thread_init(), so that we can later migrate to
glib 2.34. It also replaces dbus_g_thread_init(), which calls the
deprecated g_thread_supported(), with dbus_threads_init_default()
directly.

BUG=chromium:253025
TEST=Tested the following:
1. Build and run unit tests.
2. Run trybot builds on x86, amd64, and arm platforms.

Change-Id: I30852f1d3525f3d7b5b6cd756d18c2816a28903f
Reviewed-on: https://gerrit.chromium.org/gerrit/59801
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/testrunner.cc b/testrunner.cc
index aa75495..139de75 100644
--- a/testrunner.cc
+++ b/testrunner.cc
@@ -19,8 +19,7 @@
 int main(int argc, char **argv) {
   LOG(INFO) << "started";
   ::g_type_init();
-  g_thread_init(NULL);
-  dbus_g_thread_init();
+  dbus_threads_init_default();
   base::AtExitManager exit_manager;
   // TODO(garnold) temporarily cause the unittest binary to exit with status
   // code 2 upon catching a SIGTERM. This will help diagnose why the unittest