Add policy to use system default printer

chrome/test/data/policy/policy_test_cases.json

 {
   "-- Template --": {
     "intro": "Top-level entries map a policy name to its test parameters, described below. The name of the top level entry should be of the form <policy name>[.suffix]. The optional suffix is used for defining multiple test cases for a single policy.",
 
     "os": ["List of operating systems that support this policy. Valid values:", "win", "linux", "mac", "chromeos", "android", "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 1758 matching lines @@
   "DefaultPrinterSelection": {
     "os": ["win", "linux", "mac", "chromeos"],
     "test_policy": {
       "DefaultPrinterSelection": "{ 'kind': 'local', 'namePattern': '.*' }"
     },
     "pref_mappings": [
       { "pref": "printing.default_destination_selection_rules" }
     ]
   },
 
+  "UseSystemDefaultPrinterAsDefault": {
+    "os": ["win", "mac", "linux"],

[Lei Zhang, 2017/06/15 18:26:16]

skau: With CUPS on ChromeOS, does ChromeOS now have the concept of a default
system printer?

[skau, 2017/06/15 19:41:31]

No, we don't maintain a system default printer in CrOS atm

+    "test_policy": { "UseSystemDefaultPrinterAsDefault": false },
+    "pref_mappings": [
+      { "pref": "printing.use_system_default_printer" }
+    ]
+  },
+
   "BackgroundModeEnabled": {
     "os": ["win", "linux"],
     "can_be_recommended": true,
     "test_policy": { "BackgroundModeEnabled": false },
     "pref_mappings": [
       { "pref": "background_mode.enabled",
         "local_state": true,
         "indicator_tests": [
           { "policy": { "BackgroundModeEnabled": false } }
         ]
@@ skipping 1318 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }