Take ownership of TLS-1.3-related flags.

I've left expiry milestones alone for now. tls13-variant should
hopefully be unwound quite soon. enforce-tls13-downgrade will depend a
lot on how long it takes to clear through the mess of non-compliant
Enterprise middleboxes out there.

Bug: none
Change-Id: Iecdfd86618de36a2ba949f3eb472010fff3d5f24
Reviewed-on: https://chromium-review.googlesource.com/c/1344578Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609815}

chrome/browser/flag-metadata.json

@@ -2115,7 +2115,7 @@
   },
   {
     "name": "enforce-tls13-downgrade",
-    // "owners": [ "your-team" ],
+    "owners": [ "davidben", "svaldez" ],
     "expiry_milestone": 76
   },
   {
@@ -3050,7 +3050,7 @@
   },
   {
     "name": "tls13-variant",
-    // "owners": [ "your-team" ],
+    "owners": [ "davidben", "svaldez" ],
     "expiry_milestone": 76
   },
   {