Simplify the HTMLCollection destructor

Simplify the HTMLCollection destructor by restructuring the CollectionType
enumeration a bit and adding a isUnnamedHTMLCollectionType() convenience
function. We now use the isUnnamedHTMLCollectionType() in the HTMLCollection
destructor instead of enumerating all the "exception" types.

R=esprehn@chromium.org,adamk@chromium.org

Review URL: https://codereview.chromium.org/240273005

git-svn-id: svn://svn.chromium.org/blink/trunk@171884 bbb929c8-8fbe-4397-9dbb-9b2b20218538