Add kbr as per-file owner of system_info_handler.

This was added in support of the Telemetry-based GPU tests and I
should be able to review changes to this file and its header.

Bug: 985486, 984780, angleproject:3701
Change-Id: Ib346b273b8e646b43747b081bf14002f3494be3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761159Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688322}

content/browser/devtools/protocol/OWNERS

+per-file system_info_handler.*=kbr@chromium.org
+
+# COMPONENT: Platform>DevTools