Add flag metadata for Crostini

Assigns owners for the following Crostini flags:
crostini-usb-support
disable-crostini-files
enable-experimental-crostini-ui

Bug: 897809
Change-Id: I077743a15cb65e552d38e23f189401193723006f
Reviewed-on: https://chromium-review.googlesource.com/c/1345673Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Josh Pratt <jopra@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611021}

chrome/browser/flag-metadata.json

@@ -453,7 +453,7 @@
   },
   {
     "name": "crostini-usb-support",
-    // "owners": [ "your-team" ],
+    "owners": [ "jopra", "nverne", "benwells" ],
     "expiry_milestone": 76
   },
   {
@@ -523,7 +523,7 @@
   },
   {
     "name": "disable-crostini-files",
-    // "owners": [ "your-team" ],
+    "owners": [ "joelhockey", "nverne", "benwells" ],
     "expiry_milestone": 76
   },
   {
@@ -1143,7 +1143,7 @@
   },
   {
     "name": "enable-experimental-crostini-ui",
-    // "owners": [ "your-team" ],
+    "owners": [ "nverne", "benwells" ],
     "expiry_milestone": 76
   },
   {