Deprecate invoking the default action for untrusted events.

third_party/WebKit/Source/core/frame/Deprecation.cpp

Index: third_party/WebKit/Source/core/frame/Deprecation.cpp
diff --git a/third_party/WebKit/Source/core/frame/Deprecation.cpp b/third_party/WebKit/Source/core/frame/Deprecation.cpp
index ab1e9519d4a06dcf51cc57e8ac4aa7ec9f8b28ec..edcb99a48ae455d65352e6a16204d8540348ca21 100644
--- a/third_party/WebKit/Source/core/frame/Deprecation.cpp
+++ b/third_party/WebKit/Source/core/frame/Deprecation.cpp
@@ -388,6 +388,9 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature)
     case UseCounter::MediaStreamOnEnded:
         return replacedWillBeRemoved("The MediaStream 'ended' event", "the MediaStream 'inactive' event", 53, "5730404371791872");
 
+    case UseCounter::UntrustedEventDefaultHandled:
+        return String::format("A DOM event generated from JavaScript has triggered a default action inside the browser. This behavior is non-standard and will be removed in %s. See https://www.chromestatus.com/features/5718803933560832 for more details.", milestoneString(53));
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();