Teach WebURLRequest about Fetch's "frame type" concept.

Source/core/fetch/ResourceFetcher.cpp

Index: Source/core/fetch/ResourceFetcher.cpp
diff --git a/Source/core/fetch/ResourceFetcher.cpp b/Source/core/fetch/ResourceFetcher.cpp
index e6807836c47276cb189dea637fbdf9d496f58e0d..9918b686861da72da18a5ced7b6c4787df5279f9 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -376,6 +376,7 @@ ResourcePtr<RawResource> ResourceFetcher::fetchRawResource(FetchRequest& request
 
 ResourcePtr<RawResource> ResourceFetcher::fetchMainResource(FetchRequest& request, const SubstituteData& substituteData)
 {
+    ASSERT(request.resourceRequest().frameType() != WebURLRequest::FrameTypeNone);
     if (substituteData.isValid())
         preCacheSubstituteDataForMainResource(request, substituteData);
     return toRawResource(requestResource(Resource::MainResource, request));