[Sheriff] Disable flaky test testThumbnailFetchingResult_defaultAspectRatio

Bug: 1138739
Change-Id: Iecab6daf9b8caa37abb55ca2f7d4998768dec1a6
TBR: wychen@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500312Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820968}

chrome/android/features/start_surface/internal/javatests/src/org/chromium/chrome/features/start_surface/StartSurfaceLayoutTest.java

@@ -1137,6 +1137,7 @@ public class StartSurfaceLayoutTest {
     @Test
     @MediumTest
     @CommandLineFlags.Add({BASE_PARAMS})
+    @DisabledTest(message = "https://crbug.com/1138739")
     public void testThumbnailFetchingResult_defaultAspectRatio() throws Exception {
         prepareTabs(2, 0, mUrl);
         int oldJpegCount = RecordHistogram.getHistogramValueCountForTesting(