Quota: Add null check

StorageQuotaClient can be null in some contexts.


BUG=346577
TEST=run the failed test locally (see the issue)

Review URL: https://codereview.chromium.org/212123005

git-svn-id: svn://svn.chromium.org/blink/trunk@170565 bbb929c8-8fbe-4397-9dbb-9b2b20218538