Have Element::ensureUniqueElementData() return a reference

Have Element::ensureUniqueElementData() return a reference instead of a pointer
as it can never return null. It makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/176883005

git-svn-id: svn://svn.chromium.org/blink/trunk@167962 bbb929c8-8fbe-4397-9dbb-9b2b20218538