[NTP] Adds an experiment for M69 NTP features.

Only the first experiment entry gets covered by perf waterfall, so adding
this item ensures better test coverage. Also, the existing experiment,
NewTabPageAllBirthday_Enabled, could not be re-used since
go/revising-finch-configs recommends creating a new experiment when
changing size or behavior of existing groups.

Bug: 831910
Change-Id: I95ebc6f2768fe25d34993c05c71e35c22e2bdcc5
Reviewed-on: https://chromium-review.googlesource.com/1160925Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ramya Nagarajan <ramyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580314}

testing/variations/fieldtrial_testing_config.json

@@ -2245,6 +2245,19 @@
                 "windows"
             ],
             "experiments": [
+                {
+                    "//0": "This experiment is used for M69 release rollouts.",
+                    "//1": "The name change is required to update Canary, Dev",
+                    "//2": "and Beta Finch studies that previously used ",
+                    "//3": "NewTabPageAllBirthday_Enabled.",
+                    "name": "NewTabPageM69AllBirthday_Enabled",
+                    "enable_features": [
+                        "NewTabPageBackgrounds",
+                        "NewTabPageCustomLinks",
+                        "NewTabPageIcons",
+                        "NewTabPageUIMd"
+                    ]
+                },
                 {
                     "name": "NewTabPageM68AllBirthday_Enabled",
                     "enable_features": [
@@ -2253,6 +2266,7 @@
                     ]
                 },
                 {
+                    "//0": "Deprecated.",
                     "name": "NewTabPageAllBirthday_Enabled",
                     "enable_features": [
                         "NewTabPageBackgrounds",