Added the SuppressUnsupportedOSWarning policy setting.

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", "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 1863 matching lines @@
   "WelcomePageOnOSUpgradeEnabled": {
     "os": ["win"],
     "test_policy": { "WelcomePageOnOSUpgradeEnabled": false },
     "pref_mappings": [
       { "pref": "browser.welcome_page_on_os_upgrade_enabled",
         "local_state": true
       }
     ]
   },
 
+  "SuppressUnsupportedOSWarning": {
+    "os": ["chromeos", "linux", "mac", "win"],
+    "test_policy": { "SuppressUnsupportedOSWarning": true },
+    "pref_mappings": [
+      { "pref": "browser.suppress_unsupported_os_warning",
+        "local_state": true
+      }
+    ]
+  },
+
   "----- Chrome OS policies ------------------------------------------------": {},
 
   "ChromeOsLockOnIdleSuspend": {
     "os": ["chromeos"],
     "can_be_recommended": true,
     "test_policy": { "ChromeOsLockOnIdleSuspend": true },
     "pref_mappings": [
       { "pref": "settings.enable_screen_lock",
         "indicator_tests": [
           { "policy": { "ChromeOsLockOnIdleSuspend": true } }
@@ skipping 806 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }