DevTools: Fix content_shell devtools built in debug_devtools=1 mode

This has never worked properly, as content_shell uses both raw
(non-concatenated) JS resources and a grd-based resource bundle.
As a side effect, the flattening of build-copied resources (effectively,
cm/*.css only) and the buildSystemOnly.js file have been eliminated.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/283833006

git-svn-id: svn://svn.chromium.org/blink/trunk@174145 bbb929c8-8fbe-4397-9dbb-9b2b20218538