Disable SyncExponentialBackoffTest.OfflineToOnline on CrOS

It's very flaky.

Bug: 1170609
Change-Id: I1220d79f3b74db9b66329c0b602ac062b1cc0912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2667350
Auto-Submit: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849593}

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

@@ -67,7 +67,13 @@ class ExponentialBackoffChecker : public SingleClientStatusChangeChecker {
   DISALLOW_COPY_AND_ASSIGN(ExponentialBackoffChecker);
 };
 
-IN_PROC_BROWSER_TEST_F(SyncExponentialBackoffTest, OfflineToOnline) {
+// Flaky on ChromeOS, crbug.com/1170609
+#if BUILDFLAG(IS_CHROMEOS_ASH)
+#define MAYBE_OfflineToOnline DISABLED_OfflineToOnline
+#else
+#define MAYBE_OfflineToOnline OfflineToOnline
+#endif
+IN_PROC_BROWSER_TEST_F(SyncExponentialBackoffTest, MAYBE_OfflineToOnline) {
   const std::string kFolderTitle1 = "folder1";
   const std::string kFolderTitle2 = "folder2";