Have ElementRareData::ensure*() methods return references

Have ElementRareData::ensure*() methods return references instead of pointers
for consistency and clarity.

R=adamk@chromium.org, adamk

Review URL: https://codereview.chromium.org/183763027

git-svn-id: svn://svn.chromium.org/blink/trunk@168416 bbb929c8-8fbe-4397-9dbb-9b2b20218538