DevTools: fix remote debugging mode

In the remote debugging mode, DevTools lacks some build-generated files,
such as InspectorBackendCommand and SupportedCSSProperties.

As these files are now part of sdk/module.json file, Runtime attempts
to load them and fails in remote debugging mode.

R=pfeldman, apavlov

Review URL: https://codereview.chromium.org/604813002

git-svn-id: svn://svn.chromium.org/blink/trunk@182693 bbb929c8-8fbe-4397-9dbb-9b2b20218538