[Paint Holding] Extend flag until M-86

Bug: 1012732
Change-Id: Ica20fad07c81110b3353f0872e34b881f7a55565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068056
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743724}

chrome/browser/flag-metadata.json

@@ -1639,7 +1639,7 @@
     {
     "name": "enable-paint-holding",
     "owners": [ "schenney", "paint-dev" ],
-    "expiry_milestone": 82
+    "expiry_milestone": 86
   },
   {
     "name": "enable-parallel-downloading",