Fix Windows version in chromium.webrtc.fyi: 8.1 rather than 8

Problem introduced in https://chromium-review.googlesource.com/c/chromium/src/+/1672849

TBR: martiniss@chromium.org
Bug: 976184
Change-Id: Ib09c1f8720fab8ebdb74d67d87a0fd9db2273d04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675427Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672006}

infra/config/cr-buildbucket.cfg

@@ -4739,7 +4739,7 @@ buckets {
     builders {
       name: "WebRTC Chromium FYI Win8 Tester"
       mixins: "win"
-      dimensions: "os:Windows-8"
+      dimensions: "os:Windows-8.1"
     }
   }
 }