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..6329909f5df37174afe1a977f4ba83daf8ffcdac 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 willBeRemoved("Performing default actions on untrusted events", 53, "5718803933560832");

[Rick Byers, 2016/05/17 17:34:28]

I'm a little worried that developers may see this even when they're doing
nothing wrong (eg. there happens to be a default action as a side effect of an
event they're generating but they don't rely on it).  How about changing the
wording to something like this:

"A DOM event generated from JavaScript has triggered a default action inside the
browser.  This behavior is non-standard and will be removed in M53.  See ...".?

[dtapuska, 2016/05/17 19:32:42]

Done.

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