NUX Onboarding: clean up build process.

This CL cleans up the remnants of the NUX Experiments bulid configs. Including:
- removing os_win and google_chrome_branded build flag.
- removing prefs that tracks if the one-off NUX experiments were seen.
- fixing closure compile errors now that it needs to build on non-official bots.

Bug: 874153
Change-Id: I03c1ed09c54f5ba91944bcb31837e80a0ee1fc74
Commit-Queue: Scott Chen <>
Reviewed-by: Scott Violet <>
Cr-Commit-Position: refs/heads/master@{#596432}
18 files changed