Add sample extension for preference access.

chrome/common/extensions/docs/samples.json

 {
   "api": {
     "chrome.bookmarks.create": "bookmarks.html#method-create",
     "chrome.bookmarks.export": "bookmarks.html#method-export",
     "chrome.bookmarks.get": "bookmarks.html#method-get",
     "chrome.bookmarks.getChildren": "bookmarks.html#method-getChildren",
     "chrome.bookmarks.getRecent": "bookmarks.html#method-getRecent",
     "chrome.bookmarks.getTree": "bookmarks.html#method-getTree",
     "chrome.bookmarks.import": "bookmarks.html#method-import",
     "chrome.bookmarks.move": "bookmarks.html#method-move",
@@ skipping 581 matching lines @@
       "source_files": [
         "background.html",
         "content_script.js",
         "manifest.json",
         "options.html"
       ],
       "source_hash": "54b5469031ddcb2097f39dbaae1bcd81ae650154",
       "zip_path": "examples/extensions/email_this_page.zip"
     },
     {
+      "api_calls": [
+        "chrome.extension.isAllowedIncognitoAccess"
+      ],
+      "description": "Sample extension which demonstrates how to access a preference.",
+      "features": [
+        "browser_action",
+        "contentSettings",
+        "experimental",
+        "popup"
+      ],
+      "icon": null,
+      "id": "0bf0930a32829dfb77170535ecb27adc2de3998a",
+      "name": "Enable/disable referrers API example extension",
+      "path": "examples/api/preferences/enableReferrer/",
+      "protocols": [],
+      "search_string": "ENABLE/DISABLE REFERRERS API EXAMPLE EXTENSION SAMPLE EXTENSION WHICH DEMONSTRATES HOW TO ACCESS A PREFERENCE. BROWSER_ACTION CONTENTSETTINGS EXPERIMENTAL POPUP CHROME.EXTENSION.ISALLOWEDINCOGNITOACCESS",
+      "source_files": [
+        "manifest.json",
+        "popup.html"
+      ],
+      "source_hash": "c8b3adc6d8f9a2d24143f40ee24ec0ee9d22e09d",
+      "zip_path": "examples/api/preferences/enableReferrer.zip"
+    },
+    {
       "api_calls": [],
       "description": "A sample extension which uses Google Analytics to track usage.",
       "features": [
         "background_page",
         "browser_action",
         "popup"
       ],
       "icon": "analytics-extension-icon-128.png",
       "id": "763a08e9b06595d785568a8d392b95a2f3700258",
       "name": "Event Tracking with Google Analytics",
@@ skipping 1098 matching lines @@
       "source_files": [
         "manifest.json",
         "popup.html",
         "sample.css"
       ],
       "source_hash": "c8bfc2b60f25c24dc3c5aaf75dd635c9419df66d",
       "zip_path": "examples/api/extension/isAllowedAccess.zip"
     }
   ]
 }