service worker: Check for null weak ptr in ServiceWorkerScriptCacheMap.

context_ can be null.

No test added since there's no script cache map unittest.

Bug: 877360
Change-Id: Ib95d89255b0706d13cb784d812cb984903aa0c4a
Reviewed-on: https://chromium-review.googlesource.com/1188007
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585737}

content/browser/service_worker/service_worker_script_cache_map.cc

@@ -92,12 +92,18 @@ void ServiceWorkerScriptCacheMap::WriteMetadata(
     const GURL& url,
     const std::vector<uint8_t>& data,
     const net::CompletionCallback& callback) {
+  if (!context_) {
+    callback.Run(net::ERR_ABORTED);
+    return;
+  }
+
   ResourceMap::iterator found = resource_map_.find(url);
   if (found == resource_map_.end() ||
       found->second.resource_id == kInvalidServiceWorkerResourceId) {
     callback.Run(net::ERR_FILE_NOT_FOUND);
     return;
   }
+
   scoped_refptr<net::IOBuffer> buffer(new net::IOBuffer(data.size()));
   if (data.size())
     memmove(buffer->data(), &data[0], data.size());