Enable MSVC warning for unused locals.

cloud_print/service/win/cloud_print_service.cc

Index: cloud_print/service/win/cloud_print_service.cc
diff --git a/cloud_print/service/win/cloud_print_service.cc b/cloud_print/service/win/cloud_print_service.cc
index 55652692f3bd636d8fd5610cc4634ef2343aa4b8..182ff2dc1b42a27f15fb1bb8debaf5b0cf74974f 100644
--- a/cloud_print/service/win/cloud_print_service.cc
+++ b/cloud_print/service/win/cloud_print_service.cc
@@ -2,7 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Work around warning in atlbase.h
+// https://connect.microsoft.com/VisualStudio/feedback/details/1032199/atlbase-h-gives-warning-c4189-when-compiling-with-atl-no-com-support
+#pragma warning(push)
+#pragma warning(disable:4189)
 #include <atlbase.h>
+#pragma warning(pop)
 #include <security.h>
 
 #include <iomanip>
@@ -334,9 +339,11 @@ class CloudPrintServiceModule
 
     std::string contents;
     ServiceState service_state;
+    if (!base::ReadFileToString(file, &contents) ||
+        !service_state.FromString(contents)) {
+      return ReportError(E_FAIL, IDS_ERROR_FAILED_CREATE_CONFIG);
+    }
 
-    bool is_valid = base::ReadFileToString(file, &contents) &&
-                    service_state.FromString(contents);
     std::string proxy_id = service_state.proxy_id();
 
     LOG(INFO) << file.value() << ": " << contents;