Mark FontFaceCacheTest.MatchCombinations as flaky.

BUG=871812
TBR=rjwright@chromium.org

Change-Id: I64dc5078e3f92c18fb95b7eec1e664798bd18d26
Reviewed-on: https://chromium-review.googlesource.com/1165315Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581257}

third_party/blink/renderer/core/css/font_face_cache_test.cc

@@ -222,7 +222,8 @@ FontSelectionRange ExpectedRangeForChoice(
   }
 }
 
-TEST_F(FontFaceCacheTest, MatchCombinations) {
+// Flaky; https://crbug.com/871812
+TEST_F(FontFaceCacheTest, FLAKY_MatchCombinations) {
   CSSValue* widths[] = {CSSIdentifierValue::Create(CSSValueCondensed),
                         CSSIdentifierValue::Create(CSSValueExpanded)};
   CSSValue* slopes[] = {CSSIdentifierValue::Create(CSSValueNormal),