Add dpranke, scottmg as owners for GN-related files in src/build.

Also, remove the 'set noparent'; that was probably useful when
build/OWNERS had a wildcard, but it doesn't any more.

R=brettw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/793083002

Cr-Original-Commit-Position: refs/heads/master@{#308133}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3bce201c67c641d29f78960ebcf84b5c24467e16

config/OWNERS

@@ -1,2 +1,6 @@
-set noparent
 brettw@chromium.org
+dpranke@chromium.org
+scottmg@chromium.org
+
+per-file BUILDCONFIG.gn=brettw@chromium.org
+per-file BUILDCONFIG.gn=set noparent

toolchain/OWNERS

@@ -1,2 +1,3 @@
-set noparent
 brettw@chromium.org
+dpranke@chromium.org
+scottmg@chromium.org