* Electron version: 1.4.5
* Operating system: win10

### Expected behavior

Use about:blank without error

### Actual behavior

Devtools console will show an error message: **`GET about:blank net::ERR_UNKNOWN_URL_SCHEME`** if use `about:blank` url.

### How to reproduce

`<iframe src="about:blank">`

atom/browser/atom_browser_context.cc
}
void AtomBrowserContext::SetUserAgent(const std::string& user_agent) {
  user_agent_ = user_agent;
}
net::NetworkDelegate* AtomBrowserContext::CreateNetworkDelegate() {
  return network_delegate_;
}
net::CookieMonsterDelegate* AtomBrowserContext::CreateCookieDelegate() {