Add @RetryOnFailure to DownloadManagerServiceTest#testDownloadCompletedIsCalled

This test has flaked sometimes on Android bots.

BUG=750566
TBR=jming@chromium.org

Change-Id: I2b347ba3d055abe8b93a5a2df6a63e32d72f1a24
Reviewed-on: https://chromium-review.googlesource.com/597851Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491355}

chrome/android/javatests/src/org/chromium/chrome/browser/download/DownloadManagerServiceTest.java

@@ -406,6 +406,7 @@ public class DownloadManagerServiceTest {
     @Test
     @MediumTest
     @Feature({"Download"})
+    @RetryOnFailure
     public void testDownloadCompletedIsCalled() throws InterruptedException {
         MockDownloadNotifier notifier = new MockDownloadNotifier(getTestContext());
         MockDownloadSnackbarController snackbarController = new MockDownloadSnackbarController();