Set webrtc-unified-plan-by-default expiry milestone and owners.

With the M74 cut just around the corner, I'm setting the expiry
milestone to M75.

Bug: 938838
Change-Id: Iaa6ac07a9dd988dc07526ed8383e92c943028e8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505434Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638144}

chrome/browser/flag-metadata.json

@@ -3168,8 +3168,8 @@
   },
   {
     "name": "webrtc-unified-plan-by-default",
-    // "owners": [ "your-team" ],
-    "expiry_milestone": 76
+    "owners": [ "//third_party/blink/renderer/modules/peerconnection/OWNERS", "hbos@chromium.org" ],
+    "expiry_milestone": 75
   },
   {
     "name": "webxr",