Deprecate support for xhr.withCredentials for synchronous requests

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 494b305ccab529be3c7bb2468df4b0e6adb83ecd..097be5bcd105cf7db6b38d17a18e04a7ff41e7ad 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -734,6 +734,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case ElementSetPrefix:
         return "Setting 'Element.prefix' is deprecated, as it is read-only per DOM (http://dom.spec.whatwg.org/#element).";
 
+    case SyncXHRWithCredentials:
+        return "Setting 'XMLHttpRequest.withCredentials' for synchronous requests is deprecated.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();