[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
“Data URI Scheme” is in such format, "data:text/html,%3Chtml%3E%0A%20%20%3Chead%3E%0A%2". No way to add "file:///" as prefix. In fact, GURL is not designed for "Data URI Scheme". It only handles URL. The basic difference from URI to URL is URL is a type of URI but URL have the protocol, such as, "file://", "http://" and "ftp://". More Info, please refer to http://en.wikipedia.org/wiki/Uniform_Resource_Identifier.

So, I did a work round here to let testing pass.