Make chromeconf.h use opt-out instead of opt-in.

Code in src.git generally shouldn't be depended on by clients outside
of src.git, so that we're able to atomically change it, and so that
we're able to test changes in src.git with the CQ.

See also discussion on https://chromium-review.googlesource.com/c/chromium/src/+/1648737

Prerequisite for https://chromium-review.googlesource.com/c/chromium/src/+/1715975

No behavior change for Chromium.

Bug: 961769
Change-Id: I69e873528afc2c735b8fa28090449daae43853bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718785


Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#680934}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8efa4bab6c481bb52ca2ae9f53fab2d9734f2f6e