Disable a flaky Background Fetch browser test

FindIt identified the following test as flaky:
    BackgroundFetchBrowserTest.OfflineItemCollection_VerifyIconReceived

Let's disable it while we figure out why it's flaky.

TBR=nator
NOTRY=true

Bug: 822276
Change-Id: Ibe4ecd89cc30cdf54fda474a2bed960e599557b0
Reviewed-on: https://chromium-review.googlesource.com/964443Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543392}

chrome/browser/background_fetch/background_fetch_browsertest.cc

@@ -321,8 +321,9 @@ IN_PROC_BROWSER_TEST_F(BackgroundFetchBrowserTest,
   EXPECT_FALSE(offline_item.is_resumable);
 }
 
+// Flaky. See https://crbug.com/822276
 IN_PROC_BROWSER_TEST_F(BackgroundFetchBrowserTest,
-                       OfflineItemCollection_VerifyIconReceived) {
+                       DISABLED_OfflineItemCollection_VerifyIconReceived) {
   // Starts a Background Fetch for a single to-be-downloaded file and waits for
   // the fetch to be registered with the offline items collection. We then
   // verify that the expected icon is associated with the newly added offline