Keep the wrapper of FetchEvent alive while waiting for the preload response.

third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp

Index: third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
diff --git a/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp b/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
index a464f19d1283bdba45958c5665c665ec52c8d3cb..a87671d0f3984bf23a6e87a7511c8f895537d250 100644
--- a/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
+++ b/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
@@ -69,6 +69,15 @@ const AtomicString& FetchEvent::InterfaceName() const {
   return EventNames::FetchEvent;
 }
 
+bool FetchEvent::HasPendingActivity() const {
+  // Prevent V8 from garbage collecting the wrapper object while waiting for the
+  // preload response. This is in order to keep the resolver of preloadResponse
+  // Promise alive.
+  return preload_response_property_->GetState() ==
+             PreloadResponseProperty::kPending &&
+         GetExecutionContext();
+}
+
 FetchEvent::FetchEvent(ScriptState* script_state,
                        const AtomicString& type,
                        const FetchEventInit& initializer,
@@ -76,6 +85,7 @@ FetchEvent::FetchEvent(ScriptState* script_state,
                        WaitUntilObserver* wait_until_observer,
                        bool navigation_preload_sent)
     : ExtendableEvent(type, initializer, wait_until_observer),
+      ContextClient(ExecutionContext::From(script_state)),
       observer_(respond_with_observer),
       preload_response_property_(new PreloadResponseProperty(
           ExecutionContext::From(script_state),
@@ -109,6 +119,8 @@ FetchEvent::FetchEvent(ScriptState* script_state,
   }
 }
 
+FetchEvent::~FetchEvent() {}
+
 void FetchEvent::OnNavigationPreloadResponse(
     ScriptState* script_state,
     std::unique_ptr<WebURLResponse> response,
@@ -188,6 +200,7 @@ DEFINE_TRACE(FetchEvent) {
   visitor->Trace(request_);
   visitor->Trace(preload_response_property_);
   ExtendableEvent::Trace(visitor);
+  ContextClient::Trace(visitor);
 }
 
 }  // namespace blink