Set drive as the default download folder

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "Defaults to empty if not specified."],
     "official_only": "Whether this policy exists in official builds only. Defaults to |false| if not specified.",
     "can_be_recommended": "Whether a recommended value may be set for the policy. Defaults to |false| if not specified.",
     "test_policy": "A policy dictionary that should make the preferences affected by this policy become policy-controlled. Usually just sets the current policy. Defaults to an empty dictionary if not specified.",
     "note": "If the policy affects any preferences, the following array should be specified with one entry per such preference.",
     "pref_mappings": [
@@ skipping 395 matching lines @@
 
   "MediaCacheSize": {
     "os": ["win", "mac", "linux"],
     "test_policy": { "MediaCacheSize": 200 },
     "pref_mappings": [
       { "pref": "browser.media_cache_size" }
     ]
   },
 
   "DownloadDirectory": {
-    "os": ["win", "mac", "linux"],
+    "os": ["win", "mac", "linux", "chromeos"],
     "can_be_recommended": true,
     "test_policy": { "DownloadDirectory": "${user_home}/test-downloads" },
     "pref_mappings": [
       { "pref": "download.default_directory",
         "indicator_tests": [
           { "policy": { "DownloadDirectory": "${user_home}/test-downloads" } }
         ]
       },
       { "pref": "download.prompt_for_download",
         "indicator_tests": [
@@ skipping 1833 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }