Have LifecycleContext::lifecycleNotifier() return a reference

Have LifecycleContext::lifecycleNotifier() return a reference instead
of a pointer as it can never return null. This makes the calling
code look safer.

R=adamk
BUG=

Review URL: https://codereview.chromium.org/38143002

git-svn-id: svn://svn.chromium.org/blink/trunk@160456 bbb929c8-8fbe-4397-9dbb-9b2b20218538