Extend unified-consent expiry milestone to M79.

The unified-consent flag is still being used for ChromeOS - the current
target milestone to run the experiment is M78. This CL extends the
expiry milestone for the unified-consent flag accordingly.

Bug: None
Change-Id: I54a9697fe028e26330fb8d47c67eeb30149407e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776029Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693086}

chrome/browser/flag-metadata.json

@@ -3231,7 +3231,7 @@
   {
     "name": "unified-consent",
     "owners": [ "msarda", "tangltom" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 79
   },
   {
     "name": "unsafely-treat-insecure-origin-as-secure",