Remove old downloads UI; Material Design version is now the default.

testing/variations/fieldtrial_testing_config_chromeos.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 43 matching lines @@
     "ExtensionActionRedesign": [
         {
             "group_name": "Enabled"
         }
     ],
     "LocalNTPSuggestionsService": [
         {
             "group_name": "Enabled"
         }
     ],
-    "MaterialDesignDownloads": [

[Alexei Svitkine (slow), 2016/02/19 20:51:01]

The server-side configs still specify the trial is active.

I suggest splitting these to a separate CL and doing things in this order:

1. Land the rest of this CL that enables the new behavior as the TOT default.
2. Change the server-side experiment configs to specify max_version 49.*, so
that 50 and onward don't rely on them.
3. Land the CL that cleans up these testing configs.

-        {
-            "group_name": "Enabled"
-        }
-    ],
     "OfferUploadCreditCards": [
         {
             "group_name": "Enabled"
         }
     ],
     "PageRevisitInstrumentation": [
         {
             "group_name": "Enabled"
         }
     ],
@@ skipping 99 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }