Service Worker: Add a warning message for Client.postMessage

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 c0515e716fa740e604cd36848823bb7f439679b6..90057450e6498f8373c85633f2249a792c341bb3 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -859,6 +859,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case SVGSVGElementUnsuspendRedrawAll:
         return "'SVGSVGElement.unsuspendRedrawAll()' is deprecated, please do not use it. It is a no-op, as per SVG2 (https://svgwg.org/svg2-draft/struct.html#__svg__SVGSVGElement__unsuspendRedrawAll).";
 
+    case ServiceWorkerClientPostMessage:
+        return "'Client.postMessage()' is an experimental API and may change. See https://github.com/slightlyoff/ServiceWorker/issues/609.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();