Clarify devtools console log when a passive event listener is preventDefaulted.

third_party/WebKit/Source/core/events/Event.cpp

Index: third_party/WebKit/Source/core/events/Event.cpp
diff --git a/third_party/WebKit/Source/core/events/Event.cpp b/third_party/WebKit/Source/core/events/Event.cpp
index a66885174e79b74898ae99c6dca6cd1cc3c70724..3f4f9738ab5a41f8a1bf82f77ac5280491cbcf3d 100644
--- a/third_party/WebKit/Source/core/events/Event.cpp
+++ b/third_party/WebKit/Source/core/events/Event.cpp
@@ -80,8 +80,8 @@ Event::Event(const AtomicString& eventType, bool canBubbleArg, bool cancelableAr
     , m_cancelBubble(false)
     , m_wasInitialized(true)
     , m_isTrusted(false)
-    , m_handlingPassive(false)
     , m_preventDefaultCalledOnUncancelableEvent(false)
+    , m_handlingPassive(PassiveMode::NotPassive)
     , m_eventPhase(0)
     , m_currentTarget(nullptr)
     , m_platformTimeStamp(platformTimeStamp)
@@ -225,11 +225,26 @@ bool Event::isBeforeUnloadEvent() const
 
 void Event::preventDefault()
 {
-    if (m_handlingPassive) {
+    if (m_handlingPassive != PassiveMode::NotPassive) {
         m_preventDefaultCalledDuringPassive = true;
+
         const LocalDOMWindow* window = m_eventPath ? m_eventPath->windowEventContext().window() : 0;
-        if (window)
-            window->printErrorMessage("Unable to preventDefault inside passive event listener invocation.");
+        if (window) {
+            const char* devToolsMsg = nullptr;
+            switch (m_handlingPassive) {
+            case PassiveMode::NotPassive:
+                NOTREACHED();
+                break;
+            case PassiveMode::Passive:
+                devToolsMsg = "Unable to preventDefault inside passive event listener invocation.";
+                break;
+            case PassiveMode::PassiveForcedDocumentLevel:
+                devToolsMsg = "Unable to preventDefault inside passive event listener due to target being treated as passive. See https://www.chromestatus.com/features/5093566007214080";
+                break;
+            }
+            if (devToolsMsg)
+                window->printErrorMessage(devToolsMsg);
+        }
         return;
     }
 
@@ -281,9 +296,9 @@ HeapVector<Member<EventTarget>> Event::composedPath(ScriptState* scriptState) co
     return pathInternal(scriptState, EmptyAfterDispatch);
 }
 
-void Event::setHandlingPassive(bool value)
+void Event::setHandlingPassive(PassiveMode mode)
 {
-    m_handlingPassive = value;
+    m_handlingPassive = mode;
     m_preventDefaultCalledDuringPassive = false;
 }