Don't set Modern as default enabled in fieldtrial_testin_config.json

Add another experiment group to the Modern testing config so that Modern
isn't default enabled for tests. Enabling Modern requires restarting
Chrome twice, so having it default enabled for tests can lead to
flakiness.

BUG=

Change-Id: I822dfcdae7238782f9adf73a0ad0b65ac0a6e264
Reviewed-on: https://chromium-review.googlesource.com/1178900
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583912}

testing/variations/fieldtrial_testing_config.json

@@ -993,6 +993,12 @@
                 "android"
             ],
             "experiments": [
+                {
+                    "name": "Control",
+                    "disable_features": [
+                        "ChromeModernDesign"
+                    ]
+                },
                 {
                     "name": "Modern",
                     "enable_features": [