Allow app launch URLs to be localized.

chrome/test/data/extensions/manifest_tests/_locales/en/messages.json

+{
+  "invalid_local_launch_path": {
+    "message": "http://www.google.com/",
+    "description": "An invalid local launch path (it's an absolute URL)"
+  },
+  "local_launch_path": {
+    "message": "launch.html",
+    "description": "A valid local launch path"
+  },
+  "invalid_launch_url": {
+    "message": "javscript:evil()",
+    "description": "An invalid launch URL (not the right scheme)"
+  },
+  "launch_url": {
+    "message": "http://www.google.com/launch.html",
+    "description": "A valid launch URL"
+  }
+}