Refcount AppCacheGroup correctly.

Bug: 888926
Change-Id: Iab0d82d272e2f24a5e91180d64bc8e2aa8a8238d
Reviewed-on: https://chromium-review.googlesource.com/1246827Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594475}

content/browser/appcache/appcache_group.cc

@@ -114,9 +114,9 @@ void AppCacheGroup::AddCache(AppCache* complete_cache) {
 void AppCacheGroup::RemoveCache(AppCache* cache) {
   DCHECK(cache->associated_hosts().empty());
   if (cache == newest_complete_cache_) {
-    CancelUpdate();
     AppCache* tmp_cache = newest_complete_cache_;
     newest_complete_cache_ = nullptr;
+    CancelUpdate();
     tmp_cache->set_owning_group(nullptr);  // may cause this group to be deleted
   } else {
     scoped_refptr<AppCacheGroup> protect(this);