update_engine: Use org.chromium.NetworkProxyService.

Make update_engine call Chrome's new
org.chromium.NetworkProxyService D-Bus service to resolve
network proxies instead of using
org.chromium.LibCrosService. The new service supports
asynchronous replies instead of responding via D-Bus
signals.

BUG=chromium:446115,chromium:703217
TEST=unit tests pass; also added debug logging and verified
     that chrome's proxy settings are used

Change-Id: Iebd268ea3e551c0760416d955828b9d7ebf851fb
Reviewed-on: https://chromium-review.googlesource.com/497491
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/common/http_fetcher.cc b/common/http_fetcher.cc
index e592cc5..bcd1ffe 100644
--- a/common/http_fetcher.cc
+++ b/common/http_fetcher.cc
@@ -46,7 +46,7 @@
 }
 
 // Proxy methods to set the proxies, then to pop them off.
-bool HttpFetcher::ResolveProxiesForUrl(const string& url,
+void HttpFetcher::ResolveProxiesForUrl(const string& url,
                                        const Closure& callback) {
   CHECK_EQ(static_cast<Closure*>(nullptr), callback_.get());
   callback_.reset(new Closure(callback));
@@ -57,10 +57,10 @@
         FROM_HERE,
         base::Bind(&HttpFetcher::NoProxyResolverCallback,
                    base::Unretained(this)));
-    return true;
+    return;
   }
-  return proxy_resolver_->GetProxiesForUrl(
-      url, base::Bind(&HttpFetcher::ProxiesResolved, base::Unretained(this)));
+  proxy_resolver_->GetProxiesForUrl(url,
+      base::Bind(&HttpFetcher::ProxiesResolved, base::Unretained(this)));
 }
 
 void HttpFetcher::NoProxyResolverCallback() {