WebSessionCache: Make response parameter optional

Since I255661a62 WebSession was replaced with DynamicItem<WebSession>
in HttpRequestContext to enable plugins to provider alternative
implementations of cached web session functionality.

One side efect of it is, that now code that is using RequestScopePropagator
(with the only available implementation of GuiceRequestScopePropagator)
unable to get H2CacheBasedWebSession injected, because it relies on
ServletScopes.continueRequest() which doesn't expose HTTP response in the
continued request scope:

  return new Callable<T>() {
    public T call() throws Exception {
      checkScopingState(null == GuiceFilter.localContext.get(),
        "Cannot continue request in the same thread as a HTTP request!");
      return new GuiceFilter.Context(continuingRequest, continuingRequest,
          null)
        .call(callable);
    }
  };

Given that request scope propagation feature is primarly targeting
background tasks, make response parameter in CacheBasedWebSession
optional and adjust the code to handle this case properly.

Change-Id: Iaa5dc15f294a07b83a2411e9f29872df3a82b324

gerrit-httpd/src/main/java/com/google/gerrit/httpd/CacheBasedWebSession.java

@@ -184,6 +184,10 @@ public abstract class CacheBasedWebSession implements WebSession {
   }
 
   private void saveCookie() {
+    if (response == null) {
+      return;
+    }
+
     final String token;
     final int ageSeconds;
 

gerrit-httpd/src/main/java/com/google/gerrit/httpd/H2CacheBasedWebSession.java

@@ -17,6 +17,7 @@ package com.google.gerrit.httpd;
 import static java.util.concurrent.TimeUnit.MINUTES;
 
 import com.google.common.cache.Cache;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.registration.DynamicItem;
 import com.google.gerrit.httpd.WebSessionManager.Val;
 import com.google.gerrit.server.AnonymousUser;
@@ -56,7 +57,7 @@ public class H2CacheBasedWebSession extends CacheBasedWebSession {
   @Inject
   H2CacheBasedWebSession(
       HttpServletRequest request,
-      HttpServletResponse response,
+      @Nullable HttpServletResponse response,
       WebSessionManagerFactory managerFactory,
       @Named(WebSessionManager.CACHE_NAME) Cache<String, Val> cache,
       AuthConfig authConfig,