[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
I don't think the origin here is attempting to get the scheme. GetOrigin() in GURL will return a url containing scheme, host and port. As my understanding, here it wants to get the origin of the url of the content that attempts to use native file system API. For some reason, the instrumentation test is having an invalid url for the content. I agree with Halton, you should fix that root cause instead.