[fuchsia] Add //fuchsia/SECURITY_OWNERS and per-file OWNERS using it.

Add a set of owners for Fuchsia-specific security reviews, which will
apply to CLs which have:
- Changes to Chromium-defined FIDL APIs.
- Changes to component sandbox specifications.
- Changes to critical integration code, e.g. the sandbox policy.

The new //fuchsia/SECURITY_OWNERS file is applied to specific files in
//base, //fuchsia and //services, that are critical to the security of
WebEngine and other Chromium packages published for use on Fuchsia.

Bug: 1053551
Change-Id: I809cc07843ff06167b596f3fea21307e599803bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128079
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758932}

diff --git a/build/OWNERS.setnoparent b/build/OWNERS.setnoparent
index 9d690da..7959827 100644
--- a/build/OWNERS.setnoparent
+++ b/build/OWNERS.setnoparent

@@ -10,6 +10,7 @@
 # Security reviews
 file://chromeos/SECURITY_OWNERS
 file://content/browser/SITE_ISOLATION_OWNERS
+file://fuchsia/SECURITY_OWNERS
 file://ipc/SECURITY_OWNERS
 file://net/base/SECURITY_OWNERS
 file://sandbox/linux/OWNERS