Update DefaultPassthroughCommandDecoder experiment

Add Linux beta trial to DefaultPassthroughCommandDecoder experiment
config.

Bug: 976283
Change-Id: I01450d70116079225054bb7a9da5a1a1c6a67699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841753
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705152}

testing/variations/fieldtrial_testing_config.json

@@ -1974,11 +1974,11 @@
     "DefaultPassthroughCommandDecoder": [
         {
             "platforms": [
-                "windows"
+                "linux"
             ],
             "experiments": [
                 {
-                    "name": "Enabled",
+                    "name": "Enabled_20191004",
                     "enable_features": [
                         "DefaultPassthroughCommandDecoder"
                     ]