[base] Set base/BUILD.gn OWNERS to *.

Do not require a review from base/OWNERS for adding or removing files
from to //base and copy //content/BUILD.gn's policy from
//content/OWNERS.

R=gab@chromium.org

Change-Id: I4fea015ca56061695935c04d3256790bf5ae5c4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2682346
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#852292}

diff --git a/base/OWNERS b/base/OWNERS
index 6de5db7..9f34eac 100644
--- a/base/OWNERS
+++ b/base/OWNERS

@@ -12,6 +12,11 @@
 thestig@chromium.org
 wez@chromium.org
 
+# per-file rules:
+# These are for the common case of adding or renaming files. If you're doing
+# structural changes, please get a review from a reviewer in this file.
+per-file BUILD.gn=*
+
 # For Android-specific changes:
 per-file *android*=file://base/android/OWNERS
 per-file BUILD.gn=file://base/android/OWNERS