Use URLRequestContextBuilderMojo to create the SystemURLRequestContext.

net/url_request/url_request_context_builder_mojo.h

Index: net/url_request/url_request_context_builder_mojo.h
diff --git a/net/url_request/url_request_context_builder_mojo.h b/net/url_request/url_request_context_builder_mojo.h
index 0610864c870f27eea30e86e92f52b4e1cf12e053..29c959596f89dd206f7856dddd90806a7d72ba23 100644
--- a/net/url_request/url_request_context_builder_mojo.h
+++ b/net/url_request/url_request_context_builder_mojo.h
@@ -34,7 +34,7 @@ class URLRequestContextBuilderMojo : public URLRequestContextBuilder {
   // MojoProxyResolverFactory is provided.
   void set_dhcp_fetcher_factory(
       std::unique_ptr<DhcpProxyScriptFetcherFactory> dhcp_fetcher_factory) {
-    dhcp_fetcher_factory = std::move(dhcp_fetcher_factory_);
+    dhcp_fetcher_factory_ = std::move(dhcp_fetcher_factory);

[mmenke, 2017/06/10 01:35:33]

Definitely is a question on how much to unit test this stuff, as opposed to
relying on integration tests.  For IOThread, we also don't have many integration
tests, either.  Only two tests even test the IOThread with PAC scripts (Both of
which I wrote in the last month).

   }
 
   // Sets Mojo factory used to create ProxyResolvers. If not set, falls back to