Reland "Disable testSucessfullDownloadInIncognito on device."

This reverts commit 8c860979.

Reason for revert: Rohit sees the failures on the internal waterfall.

Original change's description:
> Revert "Disable testSucessfullDownloadInIncognito on device."
> 
> This reverts commit 0e8c98fa.
> 
> Reason for revert: Download test server performance was improved,
> which might have fixed the test. I don't have access to iPad device and won't have for a while, so reverting to see if the test still failing/flaky.
> 
> Original change's description:
> > Disable testSucessfullDownloadInIncognito on device.
> > 
> > Bug: 1059320
> > Change-Id: Ib14a8dd209e277d01b7d2769b2b7ff6da5169f7e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091937
> > Auto-Submit: Eugene But <eugenebut@chromium.org>
> > Reviewed-by: Sergio Collazos <sczs@chromium.org>
> > Commit-Queue: Sergio Collazos <sczs@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#747837}
> 
> TBR=eugenebut@chromium.org,sczs@chromium.org
> 
> # Not skipping CQ checks because original CL landed > 1 day ago.
> 
> Bug: 1059320
> Change-Id: I47716dd36214b3e2204ab2a4e1400f1b246b9310
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148168
> Auto-Submit: Eugene But <eugenebut@chromium.org>
> Reviewed-by: Sergio Collazos <sczs@chromium.org>
> Commit-Queue: Sergio Collazos <sczs@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#758806}

TBR=eugenebut@chromium.org,sczs@chromium.org

Change-Id: Ic91761c6dfa9ea731ea87dd93d7195edc7581160
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1059320
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148964Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758856}

ios/chrome/browser/ui/download/download_manager_egtest.mm

@@ -134,7 +134,15 @@ bool WaitForOpenInDownloadsButton() {
 // Tests sucessfull download up to the point where "Open in..." button is
 // presented. EarlGrey does not allow testing "Open in..." dialog, because it
 // is run in a separate process. Performs download in Incognito.
-- (void)testSucessfullDownloadInIncognito {
+#if !TARGET_IPHONE_SIMULATOR
+// TODO(crbug.com/1059320): Test consistently failing on device.
+#define MAYBE_testSucessfullDownloadInIncognito \
+  DISABLED_testSucessfullDownloadInIncognito
+#else
+#define MAYBE_testSucessfullDownloadInIncognito \
+  testSucessfullDownloadInIncognito
+#endif
+- (void)MAYBE_testSucessfullDownloadInIncognito {
   [ChromeEarlGrey openNewIncognitoTab];
   [ChromeEarlGrey loadURL:self.testServer->GetURL("/")];
   [ChromeEarlGrey waitForWebStateContainingText:"Download"];