Extend expiration for --disable-site-isolation-trials about:flags entry.

Although we now have a form of site isolation on Android, we're likely
to be doing work to expand coverage to more devices and more sites, so
it's still important to have an easily accessible opt-out switch to
check for OOPIF issues.

Change-Id: I9d9ee4fbcdbffffeda05b11b714d41918bf24d2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350493Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797081}

chrome/browser/flag-metadata.json

@@ -4072,7 +4072,7 @@
     "_comment2": "in https://crbug.com/849815.",
     "name": "site-isolation-trial-opt-out",
     "owners": [ "site-isolation-dev", "alexmos", "creis", "lukasza" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 95
   },
   {
     "name": "smart-dim-model-v3",