Fix WeakPtr bug when cache is created with a MemoryLoader.

I was writing a unittest where OpenCache is called with a MemoryLoader.
It kept crashing, and it seems to me that this was due to WriteIndex
running without callbacks. The CacheStorageCacheHandle was going out of
scope, and when a new one was being created by NotifyCacheCreated, there
was a crash due to some WeakPtr ref count internals.

This fix makes sure that the ref count is non-zero until all callbacks
consume the CacheStorageCacheHandle.

Bug: 838908
Change-Id: I610b2cad26d5608bacd18a4daa60edc976093fb4
Reviewed-on: https://chromium-review.googlesource.com/1069350Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561028}

content/browser/cache_storage/cache_storage.cc

@@ -907,13 +907,14 @@ void CacheStorage::CreateCacheDidCreateCache(
       cache_name, cache_ptr->cache_size(), cache_ptr->cache_padding(),
       cache_ptr->cache_padding_key()->key()));
 
+  CacheStorageCacheHandle handle = CreateCacheHandle(cache_ptr);
   cache_loader_->WriteIndex(
       *cache_index_,
       base::BindOnce(&CacheStorage::CreateCacheDidWriteIndex,
                      weak_factory_.GetWeakPtr(), std::move(callback),
                      CreateCacheHandle(cache_ptr)));
 
-  cache_loader_->NotifyCacheCreated(cache_name, CreateCacheHandle(cache_ptr));
+  cache_loader_->NotifyCacheCreated(cache_name, std::move(handle));
   if (cache_storage_manager_)
     cache_storage_manager_->NotifyCacheListChanged(origin_);
 }