sheriff: disable flaky SingleClientExtensionsSyncTest.StartWithSomeExtensions

Test is flaky on multiple platforms. It seems to be timing out:

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8894975161591307904/+/steps/sync_integration_tests_on__none__GPU_on_Mac__with_patch__on_Mac-10.13.6/0/logs/Flaky_failure:_SingleClientExtensionsSyncTest.StartWithSomeExtensions__status_SUCCESS_TIMEOUT_/0

TBR=mastiz@chromium.org

Bug: 1030556
Change-Id: I334eb74f5561360b1fa868ba580748d5265148f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951276Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721904}

chrome/browser/sync/test/integration/single_client_extensions_sync_test.cc

@@ -34,8 +34,9 @@ IN_PROC_BROWSER_TEST_F(SingleClientExtensionsSyncTest, StartWithNoExtensions) {
   ASSERT_TRUE(AllProfilesHaveSameExtensionsAsVerifier());
 }
 
+// Flaky: https://crbug.com/1030556
 IN_PROC_BROWSER_TEST_F(SingleClientExtensionsSyncTest,
-                       StartWithSomeExtensions) {
+                       DISABLED_StartWithSomeExtensions) {
   ASSERT_TRUE(SetupClients());
 
   const int kNumExtensions = 5;