[MIX-DL] Update field trial config for final configuration stage

This CL updates the field trial testing config to match the final stage of the MIX-DL rollout on both android and desktop platforms. Strictly speaking, we'll briefly hit a slightly different configuration in Android first, but this is the configuration that is higher risk, the equivalent config has been tested on Desktop, and the configurations don't change the code paths exercised by the field trial testing anyway.

Bug: 960819
Change-Id: I251ba5c20a30ab6532d48a22e8e6ed8eb0fee1d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548310
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@{#829171}

testing/variations/fieldtrial_testing_config.json

@@ -2530,7 +2530,7 @@
                     "name": "Enabled",
                     "params": {
                         "BlockExtensionList": "",
-                        "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",
+                        "SilentBlockExtensionList": "",
                         "TreatBlockListAsAllowlist": "false",
                         "TreatSilentBlockListAsAllowlist": "true",
                         "TreatWarnListAsAllowlist": "false",
@@ -2553,11 +2553,11 @@
                     "name": "Enabled",
                     "params": {
                         "BlockExtensionList": "",
-                        "SilentBlockExtensionList": "exe,scr,msi,vb,dmg,pkg,crx,zip,iso,rar,7z,tar,gz,bz2",
+                        "SilentBlockExtensionList": "",
                         "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"