DevTools: Make heap_snapshot_worker a top-level module

This makes all module directories consistently rooted at "front_end/".
TextUtils.js is also moved into "common", since it has nothing to do with "ui".

R=vsevik@chromium.org, pfeldman, vsevik

Review URL: https://codereview.chromium.org/407693004

git-svn-id: svn://svn.chromium.org/blink/trunk@178637 bbb929c8-8fbe-4397-9dbb-9b2b20218538