Add a use counter and deprecation message for showModalDialog

Source/core/page/UseCounter.cpp

Index: Source/core/page/UseCounter.cpp
diff --git a/Source/core/page/UseCounter.cpp b/Source/core/page/UseCounter.cpp
index e821e0f14abe231f6998f6651ae8b507fa1b0475..f6437f0459aefba27435a9291bc56b57536c7423 100644
--- a/Source/core/page/UseCounter.cpp
+++ b/Source/core/page/UseCounter.cpp
@@ -681,6 +681,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case ScrollLeftBodyNotQuirksMode:
         return "body.scrollLeft is deprecated in strict mode. Please use 'documentElement.scrollLeft' if in strict mode and 'body.scrollLeft' only if in quirks mode.";
 
+    case ShowModalDialog:
+        return "Chromium is considering deprecating showModalDialog. Please use window.open and postMessage instead.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();