Disable AudioServiceLaunchOnStartup feature in field trial on Linux.

Temporary disable feature until fixing regression reported in linked bug.

Bug: 870171
Change-Id: Ib66b1d5c820e257f2ade9cd6f492339639a241c6
Reviewed-on: https://chromium-review.googlesource.com/1164364Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Marina Ciocea <marinaciocea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581290}

testing/variations/fieldtrial_testing_config.json

@@ -409,7 +409,6 @@
     "AudioService": [
         {
             "platforms": [
-                "linux",
                 "windows"
             ],
             "experiments": [
@@ -430,6 +429,7 @@
             "platforms": [
                 "android",
                 "chromeos",
+                "linux",
                 "mac"
             ],
             "experiments": [