Oilpan: simplify quota storage callbacks objects.

With callback objects now being on the heap by default, we can simplify
the objects wrapping storage callbacks.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/676183002

git-svn-id: svn://svn.chromium.org/blink/trunk@184420 bbb929c8-8fbe-4397-9dbb-9b2b20218538