Have Document::timing() / Document::contextFeatures() return a reference

Have Document::timing() / Document::contextFeatures() return a reference
instead of a pointer as they can never return null. This makes the calling code
looks safer.

R=adamk

Review URL: https://codereview.chromium.org/181703004

git-svn-id: svn://svn.chromium.org/blink/trunk@167938 bbb929c8-8fbe-4397-9dbb-9b2b20218538