Make ElementRareData::ensureShadow() return a reference

Make ElementRareData::ensureShadow() return a reference as it can
never return null. This makes the calling code look safer.

R=tkent
BUG=

Review URL: https://codereview.chromium.org/27513004

git-svn-id: svn://svn.chromium.org/blink/trunk@159819 bbb929c8-8fbe-4397-9dbb-9b2b20218538