Add field trial testing config for IncreaseInputAudioBufferSize study.

This enables the feature with the same name.

Bug: 830624, 826664
Change-Id: I156e197bc4a5f0a322747ad13def5ef361d3b173
Reviewed-on: https://chromium-review.googlesource.com/1014083
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568021}

testing/variations/fieldtrial_testing_config.json

@@ -1843,6 +1843,21 @@
             ]
         }
     ],
+    "IncreaseInputAudioBufferSize": [
+        {
+            "platforms": [
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "IncreaseInputAudioBufferSize"
+                    ]
+                }
+            ]
+        }
+    ],
     "InstanceID": [
         {
             "platforms": [