GMC: Don't enable Picture-in-picture button for ChromeOS

Since we are no longer launching the Zenith PiP button on ChromeOS,
this CL updates the fieldtrial_testing_config.json to reflect that.

Bug: 1058771
Change-Id: I4d21021bc1db0d3ddedefd28a6c73b5c8c2fec91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091213Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747786}

testing/variations/fieldtrial_testing_config.json

@@ -2358,8 +2358,7 @@
             "platforms": [
                 "linux",
                 "mac",
-                "windows",
-                "chromeos"
+                "windows"
             ],
             "experiments": [
                 {