Declare Chrome OS Account Manager for field trial testing

Bug: 816954
Change-Id: I4ee640075af0948cbb8b5bfcc127123348cad98b
Reviewed-on: https://chromium-review.googlesource.com/c/1426779
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629940}

testing/variations/fieldtrial_testing_config.json

@@ -1741,6 +1741,21 @@
             ]
         }
     ],
+    "EnableChromeOSAccountManager": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ChromeOSAccountManager"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableCsrssLockdown": [
         {
             "platforms": [