Added stable testing fieldtrial config for the heuristic language model.

This is required to roll out the heuristic language model to 1% on stable.

Bug: 762352
Change-Id: Iba29ae1e9335b38e853f46abd5e267f16705c786
Reviewed-on: https://chromium-review.googlesource.com/894743Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533144}

testing/variations/fieldtrial_testing_config.json

@@ -3936,6 +3936,12 @@
                     "enable_features": [
                         "UseHeuristicLanguageModel"
                     ]
+                },
+                {
+                    "name": "Stable_Experiment",
+                    "enable_features": [
+                        "UseHeuristicLanguageModel"
+                    ]
                 }
             ]
         }