Show a warning when using sync xhr.

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index 7ed97b314d44de04bfa93c237ab741e5e940c5a6..05b34ecbff23c6b6fa65da657531efd4b825a98c 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -807,6 +807,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case ConsoleTimelineEnd:
         return "console.timelineEnd is deprecated. Please use the console.timeEnd instead.";
 
+    case XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload:
+        return "Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check http://xhr.spec.whatwg.org/.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();