update_engine: changes for libchrome r680000 uprev

Changes applied include:
Replace arraysize by base::size.
Replace base::MessageLoop::current()->task_runner by
base::ThreadTaskRunnerHandle::Get, and
base::MessageLoopForIO::current by base::MessageLoopCurrent::IsSet.
Remove use of base::ContainsKey.
Replace base::Int{,64}ToString by base::NumberTostring.

The changes are all compatible with current libchrome r576279.

BUG=chromium:1054279
TEST=unittest

Change-Id: Ibb6027a5070e0e2d4554a6684350168542fedf5e
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2065691
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/update_engine_client.cc b/update_engine_client.cc
index a721f7a..31448ea 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -29,6 +29,7 @@
 #include <base/strings/string_number_conversions.h>
 #include <base/strings/string_split.h>
 #include <base/threading/platform_thread.h>
+#include <base/threading/thread_task_runner_handle.h>
 #include <brillo/daemons/daemon.h>
 #include <brillo/flag_helper.h>
 #include <brillo/key_value_store.h>
@@ -86,7 +87,7 @@
 
     // 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()->task_runner()->PostTask(
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE,
         base::Bind(&UpdateEngineClient::ProcessFlagsAndExit,
                    base::Unretained(this)));