[Android] Re-enable NativeFileSystem instrumentation test

Root cause:
Instrument test loads test page as "Data URI scheme". Such type of URI
dosen't have URL origin. An empty URL origin will cause crash.

Fix:
Give a default value, "file:///", to URL origin to avoid crash.

Related to XWALK-99
You are misunderstanding my point. I'm not suggest to make `fileapi::GetIsolatedFileSystemName()` accept empty origin. The issue is that the `context_url` should not return empty origin here, it should be some issue in the test cases which is loading the content wrong, (probably it's loading data directly with no url specified)