mw.inspect: decline to report module sizes when in debug mode

- In debug mode, 1 resource = 1 request, so module size is easier to determine
  via the network view of the developer tools in Firefox & WebKit browsers.
- Determining the module size in debug mode in JavaScript would require
  modifying core ResourceLoader code or re-fetching each resource with a
  separate XHR to measure its size.
- It's not worth it.

Bug: 55553
Change-Id: I73d1bb4141c1b6ed64a72fdc416048937a2e1146
(cherry picked from commit 251e4d4acf4704a8bd58492835870087e56ebbee)