Disable a flaky ParallelDownloadTest on ChromeOS.

Disable ParallelDownloadTest.ResumptionLastSliceFinished because it's
flaky on ChromeOS.

TBR=dtrainor@chromium.org, qinmin@chromium.org

Bug: 822450
Change-Id: I90d737a8397a453af90288d05c7dc6bd8eefe23f
Reviewed-on: https://chromium-review.googlesource.com/965569Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543742}

content/browser/download/download_browsertest.cc

@@ -3155,7 +3155,14 @@ IN_PROC_BROWSER_TEST_F(ParallelDownloadTest, Resumption) {
 
 // Verifies that if the last slice is finished, parallel download resumption
 // can complete.
-IN_PROC_BROWSER_TEST_F(ParallelDownloadTest, ResumptionLastSliceFinished) {
+#if defined(OS_CHROMEOS)
+// Failing on ChromeOS: https://crbug.com/822450
+#define MAYBE_ResumptionLastSliceFinished DISABLED_ResumptionLastSliceFinished
+#else
+#define MAYBE_ResumptionLastSliceFinished ResumptionLastSliceFinished
+#endif
+IN_PROC_BROWSER_TEST_F(ParallelDownloadTest,
+                       MAYBE_ResumptionLastSliceFinished) {
   // Create the received slices data, last slice is actually finished.
   std::vector<download::DownloadItem::ReceivedSlice> received_slices = {
       download::DownloadItem::ReceivedSlice(0, 1000),