Field trial testing config for StabilityDebugging

This enables testing for the "record, collect on crash, process
postmortem without sending postmortem reports" configuration.

This is a reland attempt. The previous attempt was reverted, but I don't
think it was the true culprit. See:
https://chromium-review.googlesource.com/q/Iba7b79d153a52fca73e4e943697ebcdf5332a500

Bug: 620813
Change-Id: I5d9d38fdf4e97fb95d73e1db06c4f6d40d18ce17
Reviewed-on: https://chromium-review.googlesource.com/695445Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Pierre-Antoine Manzagol (departed) <manzagop@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505714}

testing/variations/fieldtrial_testing_config.json

@@ -3122,6 +3122,25 @@
             ]
         }
     ],
+    "StabilityDebugging": [
+        {
+            "platforms": [
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "params": {
+                        "collect_postmortem": "false",
+                        "init_flush": "true"
+                    },
+                    "enable_features": [
+                        "StabilityDebugging"
+                    ]
+                }
+            ]
+        }
+    ],
     "SubresourceFilter": [
         {
             "platforms": [