Enable audio service sandbox feature on Linux test bots.

This is a reland of https://crrev.com/c/1240280 after fixing #890850 in parent
CL.

TBR=holte

Bug: 832666, 890850
Change-Id: I98cc6918842b8acfe0dae14ed176b29b92eadc4e
Reviewed-on: https://chromium-review.googlesource.com/c/1283132
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Reviewed-by: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600679}

testing/variations/fieldtrial_testing_config.json

@@ -533,14 +533,15 @@
             ],
             "experiments": [
                 {
-                    "name": "AudioProcess_LaunchOnStartup_v6",
+                    "name": "AudioProcess_LaunchOnStartup_Sandboxed_v7",
                     "params": {
                         "teardown_timeout_s": "0"
                     },
                     "enable_features": [
                         "AudioServiceAudioStreams",
                         "AudioServiceLaunchOnStartup",
-                        "AudioServiceOutOfProcess"
+                        "AudioServiceOutOfProcess",
+                        "AudioServiceSandbox"
                     ]
                 }
             ]