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/main.cc b/main.cc
index b045793..345d6e1 100644
--- a/main.cc
+++ b/main.cc
@@ -149,8 +149,7 @@
 
 int main(int argc, char** argv) {
   ::g_type_init();
-  g_thread_init(NULL);
-  dbus_g_thread_init();
+  dbus_threads_init_default();
   base::AtExitManager exit_manager;  // Required for base/rand_util.h.
   chromeos_update_engine::Terminator::Init();
   chromeos_update_engine::Subprocess::Init();