Launch DefaultPassthroughCommandDecoder experiment on Linux

This experiment has been tested on 1% of stable users since Apr 7, 2020.
The finch release process requires the feature to be enabled in ToT by
default when it's launched so a max_version can be set for the finch
trial.

Bug: chromium:976283
Bug: chromium:882580
Change-Id: I2b25c20d8e229a1cb47bcf7f85f6bbf0fca44a9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2221434Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Cr-Commit-Position: refs/heads/master@{#776625}

testing/variations/fieldtrial_testing_config.json

@@ -1928,21 +1928,6 @@
             ]
         }
     ],
-    "DefaultPassthroughCommandDecoder": [
-        {
-            "platforms": [
-                "linux"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled_20191004",
-                    "enable_features": [
-                        "DefaultPassthroughCommandDecoder"
-                    ]
-                }
-            ]
-        }
-    ],
     "DeprecateFtp": [
         {
             "platforms": [

ui/gl/gl_features.cc

@@ -13,7 +13,7 @@ namespace features {
 // Launched on Windows, still experimental on other platforms.
 const base::Feature kDefaultPassthroughCommandDecoder{
   "DefaultPassthroughCommandDecoder",
-#if defined(OS_WIN)
+#if defined(OS_WIN) || defined(OS_LINUX)
       base::FEATURE_ENABLED_BY_DEFAULT
 #else
       base::FEATURE_DISABLED_BY_DEFAULT