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/client_library/client_dbus.cc b/client_library/client_dbus.cc
index f16b759..4ec76c5 100644
--- a/client_library/client_dbus.cc
+++ b/client_library/client_dbus.cc
@@ -214,7 +214,7 @@
 
 bool DBusUpdateEngineClient::RegisterStatusUpdateHandler(
     StatusUpdateHandler* handler) {
-  if (!base::MessageLoopForIO::current()) {
+  if (!base::MessageLoopCurrent::IsSet()) {
     LOG(FATAL) << "Cannot get UpdateEngineClient outside of message loop.";
     return false;
   }