[MIX-DL] Update field trial testing config to next blocking configuration.

This CL updates the field trial testing config to match that of the M87 launch. Even though ToT is M88, Finch would like the M87 config prior to stable launch.

See https://blog.chromium.org/2020/02/protecting-users-from-insecure.html for full public launch plan.

Change-Id: I07c29f253eb117abd6654284656325c0b5b38d04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488705
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820936}

testing/variations/fieldtrial_testing_config.json

@@ -2564,11 +2564,11 @@
                     "name": "Enabled",
                     "params": {
                         "BlockExtensionList": "",
-                        "SilentBlockExtensionList": "exe,scr,msi,vb,dmg,pkg,crx,zip,iso,rar,7z,tar,gz,bz2",
+                        "SilentBlockExtensionList": "png,gif,jpg,jpeg,tif,tiff,bmp,svg,webp,ico,mp3,wav,wma,m3u,ogg,oga,aiff,m4a,aac,flac,m4b,webm,mp4,mov,wmv,avi,txt",
                         "TreatBlockListAsAllowlist": "false",
-                        "TreatSilentBlockListAsAllowlist": "false",
-                        "TreatWarnListAsAllowlist": "true",
-                        "WarnExtensionList": "png,gif,jpg,jpeg,tif,tiff,bmp,svg,webp,ico,mp3,wav,wma,m3u,ogg,oga,aiff,m4a,aac,flac,m4b,webm,mp4,mov,wmv,avi,txt"
+                        "TreatSilentBlockListAsAllowlist": "true",
+                        "TreatWarnListAsAllowlist": "false",
+                        "WarnExtensionList": ""
                     },
                     "enable_features": [
                         "TreatUnsafeDownloadsAsActive"
@@ -2587,11 +2587,11 @@
                     "name": "Enabled",
                     "params": {
                         "BlockExtensionList": "",
-                        "SilentBlockExtensionList": "exe,scr,msi,vb,dmg,pkg,crx",
+                        "SilentBlockExtensionList": "exe,scr,msi,vb,dmg,pkg,crx,zip,iso,rar,7z,tar,gz,bz2",
                         "TreatBlockListAsAllowlist": "false",
                         "TreatSilentBlockListAsAllowlist": "false",
-                        "TreatWarnListAsAllowlist": "false",
-                        "WarnExtensionList": "zip,iso,rar,7z,tar,gz,bz2"
+                        "TreatWarnListAsAllowlist": "true",
+                        "WarnExtensionList": "png,gif,jpg,jpeg,tif,tiff,bmp,svg,webp,ico,mp3,wav,wma,m3u,ogg,oga,aiff,m4a,aac,flac,m4b,webm,mp4,mov,wmv,avi,txt"
                     },
                     "enable_features": [
                         "TreatUnsafeDownloadsAsActive"