sheriff: Disable flaky FeedNewTabPageTest#testSignInPromo

TBR=twellington@chromium.org

Bug: 1136923
Change-Id: Ideaca2e918c8e70ea42cebd9011b858be344439e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462949Reviewed-by: Lijin Shen <lazzzis@google.com>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#815712}

chrome/android/feed/core/javatests/src/org/chromium/chrome/browser/feed/v1/FeedNewTabPageTest.java

@@ -193,6 +193,7 @@ public class FeedNewTabPageTest {
     @MediumTest
     @Feature({"FeedNewTabPage"})
     @Features.DisableFeatures(ChromeFeatureList.INTEREST_FEED_V2)
+    @DisabledTest(message = "Flaky -- crbug.com/1136923")
     public void testSignInPromo() {
         openNewTabPage();
         SignInPromo.SigninObserver signinObserver = mNtp.getCoordinatorForTesting()