Update documentation to allow owners of public APIs that are used throughout the codebase to Owners-Override changes across the source tree.

Unlike LSC which is meant for work that spans dozens of CLs, this new change is meant for one-off CLs. It's similar to google3's global approvals mechanism.

base, build, blink, content and url were chosen as an initial set because they are used in many top level directories and are frequently changing.

Also add "set noparent" to top level directories to avoid inadvertent +1s from src/OWNERS bypass owners reviews sometimes.

Change-Id: Iaa397a5e09174139af21fee34dca390de132e216
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2688805
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#857391}

diff --git a/base/OWNERS b/base/OWNERS
index 9f34eac..ba026be 100644
--- a/base/OWNERS
+++ b/base/OWNERS

@@ -1,5 +1,7 @@
 # See //base/README.md to find qualification for being an owner.
 
+set noparent
+# NOTE: keep this in sync with lsc-owners-override@chromium.org owners
 ajwong@chromium.org
 danakj@chromium.org
 dcheng@chromium.org
@@ -11,6 +13,7 @@
 thakis@chromium.org
 thestig@chromium.org
 wez@chromium.org
+# NOTE: keep this in sync with lsc-owners-override@chromium.org owners
 
 # per-file rules:
 # These are for the common case of adding or renaming files. If you're doing