Add OWNERS to external/wpt/native-file-system/

This should allow FindIt to set the right component when filing bugs for
flaky tests, and setting WPT-NOTIFY to true in OWNERS also automatically
files new bugs if a test starts failing after a WPT import.

The code in blinkpy handling WPT-NOTIFY only parses a subset of the syntax
of an OWNERS file that does not include file:// entries, so manually add a
few people rather than pointing to
content/browser/native_file_system/OWNERS.

Change-Id: I539040bf18648981da0fd2c56ff4032633ac2349
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027448
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736485}

third_party/blink/web_tests/external/wpt/native-file-system/OWNERS

+# TEAM: storage-dev@chromium.org
+# COMPONENT: Blink>Storage>FileSystem
+# WPT-NOTIFY: true
+mek@chromium.org
+stevebe@microsoft.com