Allow features to refer to subkeys in _manifest_features.json.

chrome/common/extensions/docs/samples.json

 {
   "api": {
     "chrome.bookmarks.create": "bookmarks.html#method-create",
     "chrome.bookmarks.get": "bookmarks.html#method-get",
     "chrome.bookmarks.getChildren": "bookmarks.html#method-getChildren",
     "chrome.bookmarks.getRecent": "bookmarks.html#method-getRecent",
     "chrome.bookmarks.getSubTree": "bookmarks.html#method-getSubTree",
     "chrome.bookmarks.getTree": "bookmarks.html#method-getTree",
     "chrome.bookmarks.move": "bookmarks.html#method-move",
     "chrome.bookmarks.onChanged": "bookmarks.html#event-onChanged",
@@ skipping 2505 matching lines @@
       "path": "examples\/api\/transientPage\/basic\/",
       "protocols": [
         "http:\/\/"
       ],
       "search_string": "TRANSIENT PAGE EXAMPLE DEMONSTRATES USAGE AND FEATURES OF THE TRANSIENT PAGE BACKGROUND_PAGE BOOKMARKS BROWSER_ACTION EXPERIMENTAL KEYBINDING TABS CHROME.BOOKMARKS.ONREMOVED CHROME.BROWSERACTION.ONCLICKED CHROME.BROWSERACTION.SETBADGETEXT CHROME.EXPERIMENTAL.ALARMS.CREATE CHROME.EXPERIMENTAL.ALARMS.ONALARM CHROME.EXPERIMENTAL.KEYBINDING.ONCOMMAND CHROME.EXPERIMENTAL.RUNTIME.ONBACKGROUNDPAGEUNLOADINGSOON CHROME.EXPERIMENTAL.RUNTIME.ONINSTALLED CHROME.EXPERIMENTAL.WEBREQUEST.ONREQUEST CHROME.EXTENSION.ONMESSAGE CHROME.EXTENSION.SENDMESSAGE CHROME.TABS.CREATE CHROME.TABS.EXECUTESCRIPT CHROME.TABS.QUERY CHROME.TABS.SENDMESSAGE",
       "source_files": [
         "background.js",
         "content.js",
         "manifest.json"
       ],
-      "source_hash": "82ef52e9b8c34ec7d30f6524e7bafec0be655c27",
+      "source_hash": "7c0a2bea158acda5c7a006af226b61a18ed91a40",
       "zip_path": "examples\/api\/transientPage\/basic.zip"
     },
     {
       "api_calls": [
         "chrome.tts.getVoices",
         "chrome.tts.speak",
         "chrome.tts.stop"
       ],
       "crx_path": null,
       "description": "Tool for developers of Chrome TTS engine extensions to help them test their engines are implementing the API correctly.",
@@ skipping 131 matching lines @@
         "manifest.json",
         "popup.html",
         "popup.js",
         "sample.css"
       ],
       "source_hash": "8e51dc3a9577065d22ce92f3c838edc5ceaf4e65",
       "zip_path": "examples\/api\/extension\/isAllowedAccess.zip"
     }
   ]
 }