Move treat_warnings_as_errors to a sub config of chromium_code.

Before https://crrev.com/c/4870473, only /WX was included in
default_warnings. It turns out adding the rest of the "treat warnings as
error" flags in default_warnings can cause some trouble for downstream
consumers of //build that use another compiler, e.g. gcc. In addition,
this is symmetrical with the no_chromium_code config, which already
added treat_warnings_as_errors as a sub config.

Change-Id: I543fc01e119df0e5b2899bbd3547d6e7c6de305e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4886782
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1200522}