Implement the infrastructure of creating WorkerFetchContext in worker global scope.

third_party/WebKit/Source/platform/loader/fetch/FetchContext.h

Index: third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
diff --git a/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h b/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
index d302289a0ac2ef50bc0465e4c8ddbf27da00b688..19e0b11b1df6707666c0297a8263784401e858e3 100644
--- a/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
+++ b/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
@@ -52,6 +52,7 @@ class ResourceError;
 class ResourceResponse;
 class ResourceTimingInfo;
 class WebTaskRunner;
+class WebURLLoader;
 enum class WebCachePolicy;
 
 enum FetchResourceType { kFetchMainResource, kFetchSubresource };
@@ -201,6 +202,8 @@ class PLATFORM_EXPORT FetchContext
     return platform_probe_sink_;
   }
 
+  virtual WebURLLoader* CreateURLLoader() { return nullptr; }
+
  protected:
   FetchContext();