Reapply r159780 "Remove needless comments from ScriptPromise"
Reapply r159742 "generalize CallbacksPromiseAdapter"

TBR=abarth

Review URL: https://codereview.chromium.org/28783004

git-svn-id: svn://svn.chromium.org/blink/trunk@159946 bbb929c8-8fbe-4397-9dbb-9b2b20218538