Enforce the 'requirements' field in manifests.

chrome/test/data/extensions/manifest_tests/init_valid_app_requirements.json

 {
   "name": "Requirements Hosted App",
   "version": "0",
   "app": {
     "launch": {
       "web_url": "http://example.com"
     },
     "urls": [
       "http://*.example.com"
     ]
   },
   "requirements": {
-    "webgl": {
-      "extensions": ["l33t OpenGL extension"]
+    "3D": {
+      "features": ["webgl"]
     }
   }
 }