[Extensions Bindings] Introduce a supportsLazyListeners property

extensions/renderer/bindings/api_binding.cc

Index: extensions/renderer/bindings/api_binding.cc
diff --git a/extensions/renderer/bindings/api_binding.cc b/extensions/renderer/bindings/api_binding.cc
index e69eccab892f4b353b29b5c5192d380ccfb0b138..0c487ec1c42e61785d88c2e1f0fffcca256e0374 100644
--- a/extensions/renderer/bindings/api_binding.cc
+++ b/extensions/renderer/bindings/api_binding.cc
@@ -107,6 +107,7 @@ struct APIBinding::EventData {
             std::string full_name,
             bool supports_filters,
             bool supports_rules,
+            bool supports_lazy_listeners,
             int max_listeners,
             bool notify_on_change,
             std::vector<std::string> actions,
@@ -116,6 +117,7 @@ struct APIBinding::EventData {
         full_name(std::move(full_name)),
         supports_filters(supports_filters),
         supports_rules(supports_rules),
+        supports_lazy_listeners(supports_lazy_listeners),
         max_listeners(max_listeners),
         notify_on_change(notify_on_change),
         actions(std::move(actions)),
@@ -134,6 +136,9 @@ struct APIBinding::EventData {
   // Whether the event supports rules.
   bool supports_rules;
 
+  // Whether the event supports lazy listeners.
+  bool supports_lazy_listeners;
+
   // The maximum number of listeners this event supports.
   int max_listeners;
 
@@ -282,6 +287,7 @@ APIBinding::APIBinding(const std::string& api_name,
       const base::DictionaryValue* options = nullptr;
       bool supports_rules = false;
       bool notify_on_change = true;
+      bool supports_lazy_listeners = true;
       int max_listeners = binding::kNoListenerMax;
       if (event_dict->GetDictionary("options", &options)) {
         bool temp_supports_filters = false;
@@ -314,12 +320,20 @@ APIBinding::APIBinding(const std::string& api_name,
         bool unmanaged = false;
         if (options->GetBoolean("unmanaged", &unmanaged))
           notify_on_change = !unmanaged;
+        bool temp_supports_lazy_listeners = false;

[jbroman, 2017/07/10 15:33:02]

nit: Why is it so bad that we have to prevent "supportsLazyListeners: true" from
being specified that we force it to false (which is neither default nor what was
specified) in that case? It seems simpler to do:

if (options->GetBoolean("supportsLazyListeners", &supports_lazy_listeners)) {
  DCHECK(!supports_lazy_listeners)
      << "Don't specify supportsLazyListeners: true; it's the default.";
}

[lazyboy, 2017/07/14 00:04:42]

I like it this way though: API schema owners need not worry about lazy listeners
and extension system takes care of the default: listeners from lazy context
would become lazy automatically. The slight deviation is only for webview and we
should not expect usages of this outside of webview events.

 It seems simpler to do:

[Devlin, 2017/07/14 15:37:54]

I think jbroman@ was just saying that it seems excessive that we have a temp
variable here to guard against supports_lazy_listeners ever, ever being
specified as true (as opposed to *still* DCHECKing, but taking out the extra
temp variable).  I think it's a fair point - specifying true isn't the end of
the world, so as long as we still DCHECK (hopefully folks test! :P), it should
be fine.

[lazyboy, 2017/07/17 23:38:50]

Sure, I thought it was to consider removing the restriction, this SG.
  I think it's a fair point - specifying true isn't the end of

+        if (options->GetBoolean("supportsLazyListeners",
+                                &temp_supports_lazy_listeners)) {
+          DCHECK(!temp_supports_lazy_listeners)
+              << "Don't specify supportsLazyListeners: true; it's the default.";
+          supports_lazy_listeners = false;
+        }
       }
 
       events_.push_back(base::MakeUnique<EventData>(
           std::move(name), std::move(full_name), supports_filters,
-          supports_rules, max_listeners, notify_on_change,
-          std::move(rule_actions), std::move(rule_conditions), this));
+          supports_rules, supports_lazy_listeners, max_listeners,
+          notify_on_change, std::move(rule_actions), std::move(rule_conditions),
+          this));
     }
   }
 }
@@ -501,7 +515,8 @@ void APIBinding::GetEventObject(
   } else {
     retval = event_data->binding->event_handler_->CreateEventInstance(
         event_data->full_name, event_data->supports_filters,
-        event_data->max_listeners, event_data->notify_on_change, context);
+        event_data->supports_lazy_listeners, event_data->max_listeners,
+        event_data->notify_on_change, context);
   }
   info.GetReturnValue().Set(retval);
 }