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 d5efd2996eefbec428364cabe4e2b475c345a49d..dbb76be4175d1f46761b9f8ed029b25f27a28860 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -792,6 +792,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case ConsoleTimelineEnd:
         return "console.timelineEnd is deprecated. Please use the console.timeEnd instead.";
 
+    case XMLHttpRequestSynchronous:
+        return "Synchronous XMLHttpRequest is deprecated. Please use the async version.";

[kouhei (in TOK), 2014/10/07 23:21:18]

Is "deprecated" the message we want to show here? May be only mention about
speed?

[Mayur Kankanwadi, 2014/10/08 10:14:25]

I assumed with UseCounter::countDeprecation function coming into picture, the
current course is to inform the user that sync xhr on main thread is deprecated
(usage "not encouraged", with the view of removing functionality in the
future(5-10 yrs!!)).

+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();