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 6fbe7a9c22c6eb1ec76b32547ce99c4e7bb5a4bf..bab15ec4a33fdba67fa363f5427d0735943f9810 100644
--- a/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
+++ b/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
@@ -42,6 +42,7 @@
 #include "platform/weborigin/SecurityViolationReportingPolicy.h"
 #include "platform/wtf/Forward.h"
 #include "platform/wtf/Noncopyable.h"
+#include "public/platform/WebURLLoader.h"
 
 namespace blink {
 
@@ -214,6 +215,8 @@ class PLATFORM_EXPORT FetchContext
     return platform_probe_sink_;
   }
 
+  virtual std::unique_ptr<WebURLLoader> CreateURLLoader() { return nullptr; }
+
  protected:
   FetchContext();