viz: Turn on testing for UsePreferredIntervalForVideo on desktop.

R=holte@chromium.org

Bug: 976583
Change-Id: I7169cd35f84fee9b286e1357155fe357405b4ea9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208651
Auto-Submit: Khushal <khushalsagar@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772352}

testing/variations/fieldtrial_testing_config.json

@@ -6440,7 +6440,10 @@
     "UsePreferredIntervalForVideo": [
         {
             "platforms": [
-                "chromeos"
+                "chromeos",
+                "mac",
+                "windows",
+                "linux"
             ],
             "experiments": [
                 {