Oilpan: avoid access to to-be-swept ImageLoader object.

      If the ImageLoader update microtask holds onto an ImageLoader object
that's slated to be swept out, do not invoke its update method. Doing so
might in turn risk accessing some of its already swept&finalized
references.

Also tidy up WeakFactory field ordering for ImageLoader::Task.

R=haraken,kouhei
BUG=420515

Review URL: https://codereview.chromium.org/923443004

git-svn-id: svn://svn.chromium.org/blink/trunk@190140 bbb929c8-8fbe-4397-9dbb-9b2b20218538