fix an exception in the outline view

Fix an exception in the outline view:
```
Exception: The null object does not have a getter 'bottom'.

NoSuchMethodError: method not found: 'bottom'
Receiver: null
Arguments: [] (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/lib/outline.dart:25)
```

Also, add syntax highlighting for manifest.webapp files.

@ussuri
`visible` vs `showing` is a bit confusing, but ok.