Make the error message in WorkerThreadableLoader more developer-friendly.

third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp

Index: third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
diff --git a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
index 84f9ad9a2c51902036b057b52afdbf7c32f5053a..987696ef3699001c0d23eca1d5a306a374fc751a 100644
--- a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
+++ b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
@@ -147,7 +147,7 @@ void WorkerThreadableLoader::MainThreadBridge::mainThreadCreateLoader(PassOwnPtr
     m_mainThreadLoader = DocumentThreadableLoader::create(*document, this, request, options, resourceLoaderOptions);
     if (!m_mainThreadLoader) {
         // DocumentThreadableLoader::create may return 0 when the document loader has been already changed.
-        didFail(ResourceError(errorDomainBlinkInternal, 0, request.url().string(), "Can't create DocumentThreadableLoader"));
+        didFail(ResourceError(errorDomainBlinkInternal, 0, request.url().string(), "The parent document page has been unloaded."));
     }
 }