Have ElementData::attributeItem() return a reference

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

R=adamk, eseidel

Review URL: https://codereview.chromium.org/180723006

git-svn-id: svn://svn.chromium.org/blink/trunk@168037 bbb929c8-8fbe-4397-9dbb-9b2b20218538