Fix IncognitoDownloadLeakageTest for release.

Test was passing in debug because of fieldtrial config.
It's not read in release as per: https://source.chromium.org/chromium/chromium/src/+/master:testing/variations/README.md?originalUrl=https:%2F%2Fcs.chromium.org%2F

Enable commandline flag: UseDownloadOfflineContentProvider

Change-Id: If712caa1d7655ef8036ef15cc4b9f06373a0b293
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303123Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Ben Mason <benmason@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789337}

chrome/android/javatests/src/org/chromium/chrome/browser/incognito/IncognitoDownloadLeakageTest.java

@@ -67,7 +67,8 @@ import java.util.concurrent.TimeoutException;
 @RunWith(ParameterizedRunner.class)
 @UseRunnerDelegate(ChromeJUnit4RunnerDelegate.class)
 @EnableFeatures({ChromeFeatureList.CCT_INCOGNITO})
-@CommandLineFlags.Add({ChromeSwitches.DISABLE_FIRST_RUN_EXPERIENCE})
+@CommandLineFlags.Add({ChromeSwitches.DISABLE_FIRST_RUN_EXPERIENCE,
+        "enable-features=UseDownloadOfflineContentProvider"})
 public class IncognitoDownloadLeakageTest {
     private String mDownloadTestPage;
     private final String mDownloadedFileName = "test.gzip";