[webapi] Update tests for Network Information API to align with Chromium implementation

- Chromium Spec: http://w3c.github.io/netinfo

Impacted tests(approved): new 2, update 1, delete 11
Unit test platform: [Android]
Unit test result summary: pass 2, fail 2, block 0

Comment: type and downlinkMax don't exist in NetworkInformation,
but as spec describe, 2 attributes should exist
Why not use `assert_readonly()`?