Add a policy to re-enable deprecated web platform features.

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", "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 2060 matching lines @@
   },
 
   "NativeMessagingUserLevelHosts": {
     "os": ["win", "linux", "mac"],
     "test_policy": { "NativeMessagingUserLevelHosts": false },
     "pref_mappings": [
       { "pref": "native_messaging.user_level_hosts" }
     ]
   },
 
+  "EnableDeprecatedWebPlatformFeatures": {
+    "os": ["win", "linux", "mac", "chromeos"],

[Andrew T Wilson (Slow), 2014/05/08 11:32:45]

Why not also android?

[Mattias Nissler (ping if slow), 2014/05/09 16:38:27]

Good point, fixed.

+    "test_policy": {
+      "EnableDeprecatedWebPlatformFeatures": [ "ShowModalDialog" ]
+    },
+    "pref_mappings": [
+      { "pref": "enable_deprecated_web_platform_features" }
+    ]
+  },
+
   "----- Chrome OS device policies ---------------------------------------": {},
 
   "DevicePolicyRefreshRate": {
     "os": ["chromeos"],
     "test_policy": { "DevicePolicyRefreshRate": 300000 },
     "pref_mappings": [
       { "pref": "policy.device_refresh_rate",
         "local_state": true }
     ]
   },
@@ skipping 183 matching lines @@
 
   "AdditionalLaunchParameters": {
   },
 
   "SuppressChromeFrameTurndownPrompt": {
   },
 
   "SkipMetadataCheck": {
   }
 }