Enable fieldtrial testing for the passthrough command decoder on Mac.

BUG=982292

Change-Id: I5ec7e19d2ab3ddb7ab08e04a8db94e96c1bf1a74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225122
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780767}

testing/variations/fieldtrial_testing_config.json

@@ -2031,7 +2031,8 @@
     "DefaultPassthroughCommandDecoder": [
         {
             "platforms": [
-                "linux"
+                "linux",
+                "mac"
             ],
             "experiments": [
                 {