DevTools: Remove the "stylesheets" field from module descriptors at build time

The "stylsheets" field is not used at runtime,
so it is removed to reduce the footprint.

R=vsevik

Review URL: https://codereview.chromium.org/676993002

git-svn-id: svn://svn.chromium.org/blink/trunk@184338 bbb929c8-8fbe-4397-9dbb-9b2b20218538