update_engine: Migrate UE DBus service to chrome DBus bindings.

chromeos-dbus-bindings now generates the adaptor interface that
update_engine exposes over DBus. This interface is implemented in
dbus_service.{h,cc}, which now has a UpdateEngineService class
encapsulating all the service methods implementation.

This allows to write unit test for those methods, which are included
in this CL for all the non-trivial methods.

This CL now uses chrome's DBus bindings for the update_engine serive,
but the proxy interaction is still done using dbus-glib. The main loop
in the main.cc file is now replaced with the chromeos::Dameon, which
uses a chromeos::BaseMessageLoop instead of a GlibMessageLoop. This
causes the asynchronous interactions in the proxy side to not work,
which will be fixed in the next CL.

CQ-DEPEND=CL:290990,CL:291092,CL:293334
BUG=chromium:419827
TEST=Added unittest for all dbus_service methods. deployed and tested manually that update_engine dbus interface works.

Change-Id: I6a6d142b2ac1a61a4c3abcb927665b26114abe5c
Reviewed-on: https://chromium-review.googlesource.com/225324
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
diff --git a/subprocess.cc b/subprocess.cc
index 6282b2c..fa7e4cb 100644
--- a/subprocess.cc
+++ b/subprocess.cc
@@ -80,14 +80,14 @@
 
 }  // namespace
 
-void Subprocess::Init() {
+void Subprocess::Init(
+      chromeos::AsynchronousSignalHandlerInterface* async_signal_handler) {
   if (subprocess_singleton_ == this)
     return;
   CHECK(subprocess_singleton_ == nullptr);
   subprocess_singleton_ = this;
 
-  async_signal_handler_.Init();
-  process_reaper_.Register(&async_signal_handler_);
+  process_reaper_.Register(async_signal_handler);
 }
 
 Subprocess::~Subprocess() {