[MIX-DL] Update field trial testing config to match M87 launch stage.

See go/mix-dl-blog-post.

Bug: 960819
Change-Id: I36754c5c88b40a707e419af4daa360a06c16540c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2392753
Auto-Submit: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804420}

testing/variations/fieldtrial_testing_config.json

@@ -2160,11 +2160,11 @@
                     "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"