[Hindsight]: add fieldtrial for hindsight logging.

Bug: 1012936

Change-Id: Ib5e6e015f19472573038da0b08d3488abb349c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210073Reviewed-by: Tony Yeoman <tby@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772506}

testing/variations/fieldtrial_testing_config.json

@@ -2726,6 +2726,21 @@
             ]
         }
     ],
+    "HashedLoggingForHindsight": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "HashedLoggingForHindsight",
+                    "enable_features": [
+                        "CrOSActionStructuredMetrics"
+                    ]
+                }
+            ]
+        }
+    ],
     "HeapProfiling": [
         {
             "platforms": [