Add DefaultInIncognito param to config

Bug: 1032584
Change-Id: I2fc56e7c318d35e5cb61888a5d7429437ed1c162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209072
Commit-Queue: Sean Harrison <harrisonsean@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770135}

testing/variations/fieldtrial_testing_config.json

@@ -4192,6 +4192,9 @@
             "experiments": [
                 {
                     "name": "WithEnhancedExperiment_20200519",
+                    "params": {
+                        "DefaultInIncognito": "true"
+                    },
                     "enable_features": [
                         "ImprovedCookieControls",
                         "ImprovedCookieControlsForThirdPartyCookieBlocking",