Mark two flaky tests as flaky.

Tests are consistently (but not always) failing on Win7 builder.

Bug: 1082804
Change-Id: I92628ee5f12e4ae574291a3f225803fab5b699bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202318
Commit-Queue: Dana Fried <dfried@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768868}

third_party/blink/web_tests/TestExpectations

@@ -5936,3 +5936,6 @@ crbug.com/1071915 fast/hidpi/image-srcset-png-canvas.html [ Pass Failure ]
 # Flaky test blocking wpt import
 crbug.com/1082739 external/wpt/event-timing/dblclick.html [ Pass Failure ]
 
+# Sheriff 2020-05-14
+crbug.com/1082804 [ Win7 ] fast/forms/select/listbox-appearance-basic.html [ Pass Failure ]
+crbug.com/1082804 [ Win7 ] fast/forms/select/select-disabled-appearance.html [ Pass Failure ]