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>
Replace deprecated g_thread_create with g_thread_try_new

g_thread_create was deprecated in glib 2.32 (the current glib
we use) and we need to
update glib to 2.34.x to fix complex script shaping bugs in
Chrome's UI area ( http://crbug.com/245783 )

The CL to update glib 2.34.x is at
https://gerrit.chromium.org/gerrit/#/c/59652/

BUG=chromium:254228
TEST=1. Build and run unit tests
     2. Run trybot builds on x86, amd64 and arm

Change-Id: Ia6f58f7d18d6963fe3236bd56e819e38253a7fc8
Reviewed-on: https://gerrit.chromium.org/gerrit/60001
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Jungshik Shin (jungshik at google) <jshin@chromium.org>
Tested-by: Jungshik Shin (jungshik at google) <jshin@chromium.org>
build: disable -Werror for deprecation warnings

g_type_init is deprecated, breaking the build when using a version of
glib newer than 2.36, but we cannot remove those function calls until we
finish the upgrade to a glib newer than 2.36. Instead disable the error
and add FIXME notes to the calls that should eventually be removed.