Refactor main URLRequestContext creation to use URLRequestContextBuilder

net/url_request/url_request_context.cc

Index: net/url_request/url_request_context.cc
diff --git a/net/url_request/url_request_context.cc b/net/url_request/url_request_context.cc
index 465148c0873cefa6a4077f20ba69c436fd36e479..a8dcb7b41232fccfa293ffda7fff1c7cb650b782 100644
--- a/net/url_request/url_request_context.cc
+++ b/net/url_request/url_request_context.cc
@@ -40,6 +40,10 @@ URLRequestContext::~URLRequestContext() {
   AssertNoURLRequests();
 }
 
+URLRequestContextStorage* URLRequestContext::storage() {
+  return nullptr;
+}
+
 void URLRequestContext::CopyFrom(const URLRequestContext* other) {
   // Copy URLRequestContext parameters.
   set_net_log(other->net_log_);