Have Document::accessSVGExtensions() return a reference

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

R=adamk

Review URL: https://codereview.chromium.org/181713003

git-svn-id: svn://svn.chromium.org/blink/trunk@167959 bbb929c8-8fbe-4397-9dbb-9b2b20218538