Add [WillBeGarbageCollected] to Event.idl

Source/build/scripts/templates/EventFactory.cpp.tmpl

 {% from "macros.tmpl" import wrap_with_condition, license -%}
 {{ license() }}
 
 #include "config.h"
 #include "core/events/{{namespace}}Factory.h"
 
 #include "{{namespace}}Headers.h"
 #include "RuntimeEnabledFeatures.h"
 
 namespace WebCore {
 
-PassRefPtr<{{namespace}}> {{namespace}}Factory::create(const String& type)
+PassRefPtrWillBeRawPtr<{{namespace}}> {{namespace}}Factory::create(const String& type)
 {
 {%- for event in events %}
 {%- call wrap_with_condition(event.Conditional) %}
     if (type == "{{event|script_name}}"{% if event.RuntimeEnabled %} && RuntimeEnabledFeatures::{{event.RuntimeEnabled|lower_first}}(){% endif %})
         return {{event|cpp_name}}::create();
 {%- endcall %}
 {%- endfor %}
     return nullptr;
 }
 
 } // namespace WebCore