Rename createCallbackTask to createCrossThreadTask

The purposes of createCallbackTask and createCrossThreadTask are the same,
and thus these functions should have the same name.

BUG=390851

Review URL: https://codereview.chromium.org/380193002

git-svn-id: svn://svn.chromium.org/blink/trunk@177890 bbb929c8-8fbe-4397-9dbb-9b2b20218538