Use a RefPtr to store implementation in QualifiedName

Use a RefPtr to store implementation in QualifiedName instead of handling
ref-counting manually. This makes the code shorter / simpler.

R=eseidel, esprehn
BUG=None

Review URL: https://codereview.chromium.org/136063002

git-svn-id: svn://svn.chromium.org/blink/trunk@164998 bbb929c8-8fbe-4397-9dbb-9b2b20218538