Add a SafeSearch preference, policy and implementation.

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"],
     "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 trigger the managed banner in the settings UI. Usually just sets the current policy.",
     "settings_pages": ["A list of settings pages where the banner should be visible when this policy is set."],
     "note": "If the policy affects any preferences, the following array should be specified with one entry per such preference.",
@@ skipping 264 matching lines @@
     "settings_pages": ["chrome://settings-frame"],
     "pref_mappings": [
       { "pref": "safebrowsing.enabled",
         "indicator_tests": [
           { "policy": { "SafeBrowsingEnabled": false } }
         ]
       }
     ]
   },
 
+  "ForceSafeSearch": {
+    "pref": "settings.force_safesearch",
+    "test_policy": { "ForceSafeSearch": true },
+    "settings_pages": [],
+    "os": ["win", "linux", "mac", "chromeos"]
+  },
+
   "MetricsReportingEnabled": {
     "os": ["win", "mac", "linux"],
     "official_only": true,
     "can_be_recommended": true,
     "test_policy": { "MetricsReportingEnabled": false },
     "settings_pages": ["chrome://settings-frame"],
     "pref_mappings": [
       { "pref": "user_experience_metrics.reporting_enabled",
         "local_state": true,
         "indicator_tests": [
@@ skipping 1439 matching lines @@
     "test_policy": { "GCFUserDataDir": "${user_name}/test-frame" },
     "settings_pages": []
   },
 
   "AdditionalLaunchParameters": {
     "os": [],
     "test_policy": { "AdditionalLaunchParameters": "--enable-media-stream" },
     "settings_pages": []
   }
 }