Have ensureAttrNodeListForElement() return a reference

Have ensureAttrNodeListForElement() 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/27973004

git-svn-id: svn://svn.chromium.org/blink/trunk@160034 bbb929c8-8fbe-4397-9dbb-9b2b20218538