Update min engagement_threshold_for_flash to 8, to match the external experiment for HTML5ByDefault.

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 936 matching lines @@
             "platforms": [
                 "chromeos",
                 "linux",
                 "mac",
                 "win"
             ],
             "experiments": [
                 {
                     "name": "Enabled",
                     "params": {
-                        "engagement_threshold_for_flash": "4"
+                        "engagement_threshold_for_flash": "8"
                     },
                     "enable_features": [
                         "PreferHtmlOverPlugins"
                     ]
                 }
             ]
         }
     ],
     "HttpFormWarning": [
         {
@@ skipping 2102 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }