[Sheriff] Disable flaky BackgroundFetchBrowserTest

The test has been flaky recently:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=BackgroundFetchBrowserTest.OfflineItemCollection_VerifyResourceDownloadedWhenDownloadTotalLargerThanActualSize&testType=browser_tests

See the linked bug for details.
TBR=rayankans@chromium.org

Bug: 1161369
Change-Id: I063fa10ce855471b181ebeab4c9e4dc974c03e46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633361Reviewed-by: Anatoliy Potapchuk <apotapchuk@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Commit-Queue: Anatoliy Potapchuk <apotapchuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844083}

chrome/browser/background_fetch/background_fetch_browsertest.cc

@@ -601,7 +601,7 @@ IN_PROC_BROWSER_TEST_F(BackgroundFetchBrowserTest,
 
 IN_PROC_BROWSER_TEST_F(
     BackgroundFetchBrowserTest,
-    OfflineItemCollection_VerifyResourceDownloadedWhenDownloadTotalLargerThanActualSize) {
+    DISABLED_OfflineItemCollection_VerifyResourceDownloadedWhenDownloadTotalLargerThanActualSize) {
   // Starts a Background Fetch for a single to-be-downloaded file and waits for
   // the fetch to be registered with the offline items collection.
   std::vector<OfflineItem> items;