[GCM Driver] Add 'GCMTokenInvalidateAfterDays' to field trial

testing config.

Bug: 814376
Change-Id: Ic164c023e6da9bb462807c6177d0a63ba4cf454a
Reviewed-on: https://chromium-review.googlesource.com/993132
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548025}

testing/variations/fieldtrial_testing_config.json

@@ -1419,6 +1419,25 @@
             ]
         }
     ],
+    "GCMTokenInvalidAfterDays": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "win",
+                "chromeos",
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "GCMTokenInvalidAfterDays"
+                    ]
+                }
+            ]
+        }
+    ],
     "GaiaPasswordReuse": [
         {
             "platforms": [