update_engine_client: post tasks to task runners, not message loops

MessageLoop::PostTask and friends are deprecated and removed in
future versions of libchrome, so we should use TaskRunner::PostTask
instead. (Most MessageLoops in update_engine are actually of type
brillo::MessageLoop so this doesn't apply to most of them.)

BUG=b:37434548
TEST=emerge, run unit tests

Change-Id: Ic166cbc5e81c6cf321b4524c2698ed53beca98ba
Reviewed-on: https://chromium-review.googlesource.com/882286
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/update_engine_client.cc b/update_engine_client.cc
index ad6ded0..bb19632 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -79,7 +79,7 @@
 
     // We can't call QuitWithExitCode from OnInit(), so we delay the execution
     // of the ProcessFlags method after the Daemon initialization is done.
-    base::MessageLoop::current()->PostTask(
+    base::MessageLoop::current()->task_runner()->PostTask(
         FROM_HERE,
         base::Bind(&UpdateEngineClient::ProcessFlagsAndExit,
                    base::Unretained(this)));