Extend enable-skia-renderer flag

Work is on going to port this feature to other platforms, notably
OSX and CrOS. Extending the expiry will help developers.

Change-Id: Id24eb3e54e3db48bc4240268137ffd27761135b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284922Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Commit-Queue: Jonathan Backer <backer@chromium.org>
Auto-Submit: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785768}

chrome/browser/flag-metadata.json

@@ -2187,7 +2187,7 @@
   {
     "name": "enable-skia-renderer",
     "owners": [ "backer", "kylechar" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 90
   },
   {
     "name": "enable-speculative-service-worker-start-on-query-input",