flags: Extend double-buffer-compositing till M88

We're working toward the goal of always using double buffering
in CrOS.
This flag is useful to test Chrome with only double buffer.

Bug: None
Test: None
Change-Id: I86e3be97ee91a62742ca07c8a93a68b4c3db7cd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370426
Auto-Submit: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800853}

chrome/browser/flag-metadata.json

@@ -955,7 +955,7 @@
   {
     "name": "double-buffer-compositing",
     "owners": [ "chromeos-gfx@google.com" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 88
   },
   {
     "name": "double-tap-to-zoom-in-tablet-mode",