update_engine: Update libchrome APIS to r456626.

The new libchrome has been ported from Chromium and some APIs
have changed. Make necessary changes at call sites.

BUG=chromium:724678
CQ-DEPEND=CL:480928
Test: Build.

Change-Id: I01b70da87521d0884ed21acbd7ed3e0ff1e94357
Merged-In: I4dbaea4a2a19031375a8bf2415645a4f226dab57
diff --git a/update_engine_client.cc b/update_engine_client.cc
index 44897e0..dbfc66b 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -73,7 +73,7 @@
 
     // 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)));