flags: take ownership of //components/flags_ui

Bug: None
Change-Id: I17ed1cdbd53033429dc4b045c4225e55f838e775
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912459
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714561}

components/flags_ui/OWNERS

 file://ui/webui/PLATFORM_OWNERS
 
 asvitkine@chromium.org
+ellyjones@chromium.org
 
 # COMPONENT: UI>Browser>WebUI