Disable rendering.desktop/gmail on Windows

The story is flaky, failing >10% of the time

TBR=nednguyen@chromium.org
CC=vmiura@chromium.org, sadrul@chromium.org

Bug: 841931
Change-Id: Id949871eb6ebb87b4ffca76ebfe6d1f91cbec154
Reviewed-on: https://chromium-review.googlesource.com/1055589Reviewed-by: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557897}

tools/perf/expectations.config

@@ -115,6 +115,7 @@ crbug.com/528474 [ All ] rendering.desktop/cnn [ Skip ]
 crbug.com/762165 [ Win ] rendering.desktop/google_plus [ Skip ]
 crbug.com/762165 [ Win ] rendering.desktop/google_image_search [ Skip ]
 crbug.com/762165 [ Win ] rendering.desktop/google_docs [ Skip ]
+crbug.com/841931 [ Win ] rendering.desktop/gmail [ Skip ]
 
 # Benchmark: rendering.mobile
 crbug.com/667432 [ All ] rendering.mobile/amazon_desktop [ Skip ]