Make extensions developer mode adhere to policy

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": [
       { "pref": "The affected preference's name.",
         "local_state": "Whether |pref| is registered in local state's PrefService instead of the profile's PrefService. Defaults to |false| if not specified.",
         "check_for_mandatory": "Should the preference be tested when a mandatory value is set for the policy? Defaults to |true| if not specified.",
         "check_for_recommended": "Should the preference be tested when a recommended value is set for the policy? Defaults to |true| if not specified.",
         "note": "When |can_be_recommended| is |false|, the policy is never set to a recommended value so |check_for_recommended| has no effect.",
         "note": "The following entries should be specified if controlled setting indicators exist for |pref| in the settings UI.",
+        "indicator_test_url": "The URL to navigate to in order to test the indicators. Defaults to |chrome://settings-frame/| if not specified.",
         "indicator_test_setup_js": "Any JavaScript that should be executed before testing the indicators. This should be specified only if an explicit user action must be simulated (e.g. clicking a button).",
         "indicator_selector": "A CSS selector that locates all controlled setting indicators for |pref|. This is appended to the selector 'span.controlled-setting-indicator' and if not specified, defaults to '[pref=(the value of |pref|)', e.g. '[pref=homepage]'.",
         "note": "Any number of test cases may be specified in the following array.",
         "indicator_tests": [
           { "policy": "A policy dictionary that should affect |pref| when set as mandatory or recommended policy.",
             "value": "The value that |pref| should take on. This must only be specified if |pref| has multiple controlled setting indicators, each corresponding to a specific value (e.g. indicators next to radio buttons).",
             "readonly": "Whether setting the policy dictionary as recommended should cause |pref| to become read-only in the settings UI. This will be the case when the dictionary sets another policy that makes |pref| not applicable (e.g. setting 'homepage is NTP' makes the 'homepage URL' pref not applicable and read-only)."
           }
         ]
       }
@@ skipping 811 matching lines @@
           { "policy": { "ShowHomeButton": true } }
         ]
       }
     ]
   },
 
   "DeveloperToolsDisabled": {
     "os": ["win", "linux", "mac", "chromeos"],
     "test_policy": { "DeveloperToolsDisabled": true },
     "pref_mappings": [
-      { "pref": "devtools.disabled" }
+      { "pref": "devtools.disabled",
+        "indicator_test_url": "chrome://extensions-frame/",
+        "indicator_selector": "#dev-toggle-disabled-by-policy-indicator",
+        "indicator_tests": [
+          { "policy": { "DeveloperToolsDisabled": true } }
+        ]
+      }
     ]
   },
 
   "RestoreOnStartup": {
     "os": ["win", "linux", "mac", "chromeos"],
     "can_be_recommended": true,
     "test_policy": { "RestoreOnStartup": 4 },
     "pref_mappings": [
       { "pref": "session.restore_on_startup",
         "indicator_tests": [
@@ skipping 2176 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }