Fix StorageHandler::Disable() to not dereference-after-move()ing.

Previously the order of std::move() versus dereference to fetch the
|indexed_db_observer_|'s TaskRunner was unspecified, potentially leading
to a null dereference.

Bug: 794842
Change-Id: If08358fcf017bfebe0d615c9230f8af7a02177c9
Reviewed-on: https://chromium-review.googlesource.com/827603Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524565}

content/browser/devtools/protocol/storage_handler.cc

@@ -263,8 +263,10 @@ Response StorageHandler::Disable() {
                               cache_storage_observer_.release());
   }
   if (indexed_db_observer_) {
-    indexed_db_observer_->TaskRunner()->DeleteSoon(
-        FROM_HERE, std::move(indexed_db_observer_));
+    scoped_refptr<base::SequencedTaskRunner> observer_task_runner =
+        indexed_db_observer_->TaskRunner();
+    observer_task_runner->DeleteSoon(FROM_HERE,
+                                     std::move(indexed_db_observer_));
   }
 
   return Response::OK();