Add FilesApp owners for select_file_dialog_extension

FilesApp OWNERS should be consulted, and also help review changes, to
the ChromeOS system modal select file dialog.

Bug: 895703
Change-Id: I4258de9c2b3bfc7e37e68b546c9e16a1395572dc
Reviewed-on: https://chromium-review.googlesource.com/c/1336952
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608216}

chrome/browser/ui/views/OWNERS

@@ -13,4 +13,6 @@ per-file drag_and_drop_interactive_uitest.*=file://content/OWNERS
 
 per-file hung_renderer*=file://chrome/browser/ui/hung_renderer/OWNERS
 
+per-file select_file_dialog_extension*=file://ui/file_manager/OWNERS
+
 # COMPONENT: UI>Browser