Possible crash fix in ServiceWorker's InstallEvent::waitUntil

Depending on which argument is evaluated first |observer| 
could be NULL before we dereference observer->executionContext()

BUG=285976
TEST=content_browsertests.ServiceWorkerVersionBrowserTest.Install (to be added)

Review URL: https://codereview.chromium.org/154163004

git-svn-id: svn://svn.chromium.org/blink/trunk@166987 bbb929c8-8fbe-4397-9dbb-9b2b20218538