viz: Add param for UsePreferredIntervalForVideo for fieldtrial testing.

A delay of 6 frames is to enforce 100ms of low frame rate before
toggling global composition rate to that value.

R=asvitkine@chromium.org

Bug: 976583
Change-Id: I618a9e659062da591e366de814b049f222317880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2242529
Auto-Submit: Khushal <khushalsagar@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#778838}

testing/variations/fieldtrial_testing_config.json

@@ -7062,6 +7062,9 @@
             "experiments": [
                 {
                     "name": "Enabled",
+                    "params": {
+                        "NumOfFramesToToggleInterval": "6"
+                    },
                     "enable_features": [
                         "UsePreferredIntervalForVideo"
                     ]