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/common/error_code_utils.cc b/common/error_code_utils.cc
index 5bcbaa4..397cdf2 100644
--- a/common/error_code_utils.cc
+++ b/common/error_code_utils.cc
@@ -171,7 +171,7 @@
       // error codes which should be added here.
   }
 
-  return "Unknown error: " + base::UintToString(static_cast<unsigned>(code));
+  return "Unknown error: " + base::NumberToString(static_cast<unsigned>(code));
 }
 
 }  // namespace utils