DevTools: Get rid of a sync callback in UISourceCode#commitWorkingCopy()

This is a leftover from UISC refactorings and no longer makes sense.

R=sergeyv, vsevik

Review URL: https://codereview.chromium.org/433073002

git-svn-id: svn://svn.chromium.org/blink/trunk@179467 bbb929c8-8fbe-4397-9dbb-9b2b20218538