Add some documentation for win/cross builds, remove assert() disallowing them.

Also, in win/cross builds:
* enable clang by default
* disable NaCl by default

Bug: 495204
Change-Id: Iae3c982097970227c45450fa449f6a56d64f148f
Reviewed-on: https://chromium-review.googlesource.com/717077
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508745}
4 files changed