[ServiceWorker] Make fetch() method better conformance with the spec.

Source/core/fetch/Resource.cpp

Index: Source/core/fetch/Resource.cpp
diff --git a/Source/core/fetch/Resource.cpp b/Source/core/fetch/Resource.cpp
index 7e6112b5884ea82d26e084c7d9b5785d95194997..6f1d65a8aeeac289ce80fc799ed32233a9092266 100644
--- a/Source/core/fetch/Resource.cpp
+++ b/Source/core/fetch/Resource.cpp
@@ -342,7 +342,7 @@ static bool canUseResponse(ResourceResponse& response, double responseTimestamp)
     return currentAge(response, responseTimestamp) <= freshnessLifetime(response, responseTimestamp);
 }
 
-const ResourceRequest& Resource::lastResourceRequest()
+const ResourceRequest& Resource::lastResourceRequest() const
 {
     if (!m_redirectChain.size())
         return m_resourceRequest;