bindings: Removes two FIXMEs because setReturnValueFast is useful.

I've benchmarked and found that DOMDataStore::setReturnValueFast
is quite effective for document.implementation and
getElementsByTagName.  We should keep this method.

BUG=235436

Review URL: https://codereview.chromium.org/702523003

git-svn-id: svn://svn.chromium.org/blink/trunk@185175 bbb929c8-8fbe-4397-9dbb-9b2b20218538