Reland "Add MojoDedicatedThread field trial testing config"

This is a reland of 6eda333b

The test-only bug which elicited the revert has been fixed,
so this is relanding the original CL unmodified.

Original change's description:
> Add MojoDedicatedThread field trial testing config
>
> Bug: 1082761
> Change-Id: Ief2b018876f8cb2ad77495c7158db5c53fd8c206
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2240407
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Commit-Queue: Ken Rockot <rockot@google.com>
> Cr-Commit-Position: refs/heads/master@{#777367}

Bug: 1082761
Tbr: asvitkine@chromium.org
Change-Id: Icefe444d49199e402a772775f687374b7c14b1c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264732
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#781943}

testing/variations/fieldtrial_testing_config.json

@@ -4054,6 +4054,25 @@
             ]
         }
     ],
+    "MojoDedicatedThread": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "MojoDedicatedThread"
+                    ]
+                }
+            ]
+        }
+    ],
     "MyChromeEverywhere": [
         {
             "platforms": [