[sheriff] Disable http/tests/devtools/coverage/multiple-instances-merge.js on Win10.

Flaking a lot (flake score 4328):

https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyXAsSBUZsYWtlIlFjaHJvbWl1bUBibGlua193ZWJfdGVzdHNAaHR0cC90ZXN0cy9kZXZ0b29scy9jb3ZlcmFnZS9tdWx0aXBsZS1pbnN0YW5jZXMtbWVyZ2UuanMM

Already disabled for Win7.

TBR=caseq

Bug: 848799
Change-Id: Ic8aaf5bff039f92d83aebb2057f07b4c5109f240
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062830Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742424}

third_party/blink/web_tests/TestExpectations

@@ -6727,3 +6727,4 @@ crbug.com/1050995 [ Fuchsia ] http/tests/pointer-lock/iframe-sandboxed.html [ Ti
 crbug.com/1051774 [ Fuchsia ] http/tests/plugins/navigator-plugins-in-cross-origin-frame.html [ Timeout Pass ]
 crbug.com/1051133 [ Fuchsia ] http/tests/canvas/philip/tests/security.pattern.canvas.fillStyle.html [ Crash Timeout Pass ]
 crbug.com/1051131 [ Fuchsia ] http/tests/appcache/abort-cache-onchecking-resource-404.html [ Timeout Pass ]
+crbug.com/848799 [ Win10 ] http/tests/devtools/coverage/multiple-instances-merge.js [ Pass Timeout ]