Make member const; remove comment.

Make HttpNetworkSession::proxy_resolution_service_ member const.
Remove obsolete comment: HttpNetworkSessionPeer does not access this
member since https://crrev.com/2146553007.

Change-Id: I7decf9bec77015e0815af7ec5ac0467f91faf1d1
Reviewed-on: https://chromium-review.googlesource.com/1007156Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550109}

net/http/http_network_session.h

@@ -358,8 +358,7 @@ class NET_EXPORT HttpNetworkSession : public base::MemoryCoordinatorClient {
   CertVerifier* const cert_verifier_;
   HttpAuthHandlerFactory* const http_auth_handler_factory_;
 
-  // Not const since it's modified by HttpNetworkSessionPeer for testing.
-  ProxyResolutionService* proxy_resolution_service_;
+  ProxyResolutionService* const proxy_resolution_service_;
   const scoped_refptr<SSLConfigService> ssl_config_service_;
 
   HttpAuthCache http_auth_cache_;