Shows error messages in the inspector when .respondWith() is called with wrong responses.

Source/modules/serviceworkers/RespondWithObserver.cpp

Index: Source/modules/serviceworkers/RespondWithObserver.cpp
diff --git a/Source/modules/serviceworkers/RespondWithObserver.cpp b/Source/modules/serviceworkers/RespondWithObserver.cpp
index 0c2884931d38c5d312c58b075686ef42c5cab3ec..98b792df6a7c071dd698d8aa11de744a5e32bcbf 100644
--- a/Source/modules/serviceworkers/RespondWithObserver.cpp
+++ b/Source/modules/serviceworkers/RespondWithObserver.cpp
@@ -23,6 +23,43 @@
 #include <v8.h>
 
 namespace blink {
+namespace {
+
+// Returns the error message to let the developer know about the reason of the unusual failures.
+const String getMessageForResponseError(WebServiceWorkerResponseError error, const KURL& requestURL)
+{
+    String errorMessage = "The FetchEvent for \"" + requestURL.string() + "\" resulted in a network error response: ";
+    switch (error) {
+    case WebServiceWorkerResponseErrorPromiseRejected:
+        errorMessage = errorMessage + "the promise was rejected.";
+        break;
+    case WebServiceWorkerResponseErrorDefaultPrevented:
+        errorMessage = errorMessage + "preventDefault() was called without calling respondWith().";
+        break;
+    case WebServiceWorkerResponseErrorNoV8Instance:
+        errorMessage = errorMessage + "an object that was not a Response was passed to respondWith().";
+        break;
+    case WebServiceWorkerResponseErrorResponseTypeError:
+        errorMessage = errorMessage + "the promise was resolved with an error response object.";
+        break;
+    case WebServiceWorkerResponseErrorResponseTypeOpaque:
+        errorMessage = errorMessage + "an \"opaque\" response was used for a request whose type is not no-cors";
+        break;
+    case WebServiceWorkerResponseErrorResponseTypeNotBasicOrDefault:
+        errorMessage = errorMessage + "the response for a client request must have type \"basic\" or \"default\".";
+        break;
+    case WebServiceWorkerResponseErrorBodyUsed:
+        errorMessage = errorMessage + "a Response whose \"bodyUsed\" is \"true\" cannot be used to respond to a request.";
+        break;
+    case WebServiceWorkerResponseErrorUnknown:
+    default:
+        errorMessage = errorMessage + "an unexpected error occurred.";
+        break;
+    }
+    return errorMessage;
+}
+
+} // namespace
 
 class RespondWithObserver::ThenFunction final : public ScriptFunction {
 public:
@@ -69,9 +106,9 @@ private:
     ResolveType m_resolveType;
 };
 
-RespondWithObserver* RespondWithObserver::create(ExecutionContext* context, int eventID, WebURLRequest::FetchRequestMode requestMode, WebURLRequest::FrameType frameType)
+RespondWithObserver* RespondWithObserver::create(ExecutionContext* context, int eventID, const KURL& requestURL, WebURLRequest::FetchRequestMode requestMode, WebURLRequest::FrameType frameType)
 {
-    return new RespondWithObserver(context, eventID, requestMode, frameType);
+    return new RespondWithObserver(context, eventID, requestURL, requestMode, frameType);
 }
 
 void RespondWithObserver::contextDestroyed()
@@ -111,6 +148,8 @@ void RespondWithObserver::respondWith(ScriptState* scriptState, const ScriptValu
 void RespondWithObserver::responseWasRejected(WebServiceWorkerResponseError error)
 {
     ASSERT(executionContext());
+    executionContext()->addConsoleMessage(ConsoleMessage::create(JSMessageSource, WarningMessageLevel, getMessageForResponseError(error, m_requestURL)));
+
     // The default value of WebServiceWorkerResponse's status is 0, which maps
     // to a network error.
     WebServiceWorkerResponse webResponse;
@@ -174,9 +213,10 @@ void RespondWithObserver::responseWasFulfilled(const ScriptValue& value)
     m_state = Done;
 }
 
-RespondWithObserver::RespondWithObserver(ExecutionContext* context, int eventID, WebURLRequest::FetchRequestMode requestMode, WebURLRequest::FrameType frameType)
+RespondWithObserver::RespondWithObserver(ExecutionContext* context, int eventID, const KURL& requestURL, WebURLRequest::FetchRequestMode requestMode, WebURLRequest::FrameType frameType)
     : ContextLifecycleObserver(context)
     , m_eventID(eventID)
+    , m_requestURL(requestURL)
     , m_requestMode(requestMode)
     , m_frameType(frameType)
     , m_state(Initial)