Update enable-desktop-pwas-omnibox-install flag expiry to 79

We intend to launch PWA Omnibox install in 76 so update expiry to be 3
milestones later.

Bug: 907351
Change-Id: Ib6f59f1184f5a0a046a488fa2570d026cf5f69ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580987
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653578}

chrome/browser/flag-metadata.json

@@ -1007,7 +1007,7 @@
   {
     "name": "enable-desktop-pwas-omnibox-install",
     "owners": [ "desktop-pwas-team@google.com" ],
-    "expiry_milestone": 77
+    "expiry_milestone": 79
   },
   {
     "name": "enable-devtools-experiments",