[NTP] Creates a separate experiment for M68 NTP features.

Specifically, this removes NewTabPageUIMd (which inadvertently
enabled all UI features), and also removes NewTabPageCustomLinks
(which is only available in M69).

Bug: 831910
Change-Id: I03bc12477bba7f94576ff9a8f35f12ac3ba62299
Reviewed-on: https://chromium-review.googlesource.com/1153211Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578767}

testing/variations/fieldtrial_testing_config.json

@@ -2245,6 +2245,13 @@
                 "windows"
             ],
             "experiments": [
+                {
+                    "name": "NewTabPageM68AllBirthday_Enabled",
+                    "enable_features": [
+                        "NewTabPageBackgrounds",
+                        "NewTabPageIcons"
+                    ]
+                },
                 {
                     "name": "NewTabPageAllBirthday_Enabled",
                     "enable_features": [