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.",
 
     "pref": "The pref that this policy maps to, if any.",
     "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.",
     "indicator_tests": [
       "A list of test cases verifying that the controlled setting indicators for |pref| correctly indicate whether its value is recommended or enforced by a corresponding policy. Each test case is a dictionary with the following format:",
       { "policy": "A policy dictionary that should affect |pref| when set as mandatory or recommended policy.",
@@ skipping 174 matching lines @@
     "os": ["win", "mac", "linux"]
   },
 
   "SafeBrowsingEnabled": {
     "pref": "safebrowsing.enabled",
     "test_policy": { "SafeBrowsingEnabled": false },
     "settings_pages": ["chrome://settings-frame"],
     "os": ["win", "linux", "mac", "chromeos"]
   },
 
+  "SafeSearchEnabled": {
+    "pref": "settings.safesearch",
+    "test_policy": { "SafeSearchEnabled": true },
+    "settings_pages": [],
+    "os": ["win", "linux", "mac", "chromeos"]
+  },
+
   "MetricsReportingEnabled": {
     "pref": "user_experience_metrics.reporting_enabled",
     "test_policy": { "MetricsReportingEnabled": false },
     "settings_pages": ["chrome://settings-frame"],
     "os": ["win", "mac", "linux"],
     "local_state": true,
     "official_only": true
   },
 
   "PasswordManagerEnabled": {
@@ skipping 989 matching lines @@
     "os": []
   },
 
   "AdditionalLaunchParameters": {
     "pref": null,
     "test_policy": { "AdditionalLaunchParameters": "--enable-media-stream" },
     "settings_pages": [],
     "os": []
   }
 }