Add WebAudioAutoplay experiment to fieldtrial_testing.

Bug: 892483
Change-Id: Ia0a8347c1f31ba087759c2f53bea156f9d091d41
Reviewed-on: https://chromium-review.googlesource.com/c/1264818Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597243}

testing/variations/fieldtrial_testing_config.json

@@ -5107,6 +5107,25 @@
             ]
         }
     ],
+    "WebAudioAutoplay": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "disable_features": [
+                        "AutoplayIgnoreWebAudio"
+                    ]
+                }
+            ]
+        }
+    ],
     "WebAuthenticationUI": [
         {
             "platforms": [