Keep shared web::test::HttpServer instance alive

The web::test::HttpServer instance returned by GetSharedInstance has
zero reference count initially, HttpServer::StartOrDie implicitly
increments the reference count through base::Bind, and its
embedded_test_server_ destruction decrements the reference count.
That is, once the server stopped, the shared instance is destroyed, and
GetSharedInstance() starts returning a stale pointer.

After this CL, the resulting instance of GetSharedInstance has non-zero
reference count, so that it keeps alive after server stop.

Bug: 866456
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I1f13dff1e86d08efb28b5805ae73426b874a4f80
Reviewed-on: https://chromium-review.googlesource.com/1146881Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579314}

ios/web/public/test/http_server/http_server.mm

@@ -56,6 +56,7 @@ HttpServer& HttpServer::GetSharedInstance() {
   static dispatch_once_t once;
   dispatch_once(&once, ^{
     shared_instance = new HttpServer();
+    shared_instance->AddRef();
   });
   return *shared_instance;
 }