ASSERT that QualifiedNames are only ever created/destroyed on the main thread

I also renamed one of the local types to have a slightly longer name.

R=abarth

Review URL: https://codereview.chromium.org/25526004

git-svn-id: svn://svn.chromium.org/blink/trunk@158664 bbb929c8-8fbe-4397-9dbb-9b2b20218538