Disable smoothness.tough_webgl_cases on Windows.

It's flaking with the same error that causes it to be disabled on other platforms.

TBR=bajones@chromium.org
BUG=373812

Review URL: https://codereview.chromium.org/502133006

Cr-Commit-Position: refs/heads/master@{#291932}

tools/perf/benchmarks/smoothness.py

@@ -27,7 +27,7 @@ class SmoothnessToughCanvasCases(benchmark.Benchmark):
   page_set = page_sets.ToughCanvasCasesPageSet
 
 
-@benchmark.Disabled('android', 'mac')  # crbug.com/373812
+@benchmark.Disabled('android', 'mac', 'win')  # crbug.com/373812
 class SmoothnessToughWebGLCases(benchmark.Benchmark):
   test = smoothness.Smoothness
   page_set = page_sets.ToughWebglCasesPageSet