chromeos: Remove blanket file access logic from chromeos chrome on Linux
crrev.com/472352 removed kTestType conditional from ChromeNetworkDelegate
for Chrome OS, but this change adds it back, and instead remove blanket file
access logic based on IsRunningOnChromeOS().
This blanket file access logic was problematic when it comes to write an end-to-end
browser test that ensures access control of file: scheme is working correctly
(i.e. with this logic, browser_tests for Chrome OS that runs on Linux desktop can
access to any files).
If you need to load your files with Chrome for Chrome OS on Linux
desktop , please place these files under /tmp, that's white listed
 This build config is for development only, not for production.
TEST=access to file:///tmp is allowed on Linux desktop, but file:/// is not