update_engine: Remove all references to glib.

The message loop in production code switched from
chromeos::GlibMessageLoop to chromeos::BaseMessageLoop used in
chromeos::Dameon. This patch moves all the unittests also to use
chromeos::BaseMessageLoop or chromeos::FakeMessageLoop when I/O is not
required.

This patch removes all (now unused) references to glib.

BUG=chromium:499886
TEST=`grep glib[^c] . -R` return no results. Also, unittests work.

Change-Id: Iddfb17def501829d932ed1f70faad3e00268898e
Reviewed-on: https://chromium-review.googlesource.com/292157
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
diff --git a/p2p_manager.h b/p2p_manager.h
index fb07c2e..559bba8 100644
--- a/p2p_manager.h
+++ b/p2p_manager.h
@@ -77,7 +77,7 @@
   // Asynchronously finds a peer that serves the file identified by
   // |file_id|. If |minimum_size| is non-zero, will find a peer that
   // has at least that many bytes. When the result is ready |callback|
-  // is called from the default GLib mainloop.
+  // is called from the current message loop.
   //
   // This operation may take a very long time to complete because part
   // of the p2p protocol involves waiting for the LAN-wide sum of all