* 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
if (!guest_manager_)
    guest_manager_.reset(new WebViewManager);
  return guest_manager_.get();
}
content::PermissionManager* AtomBrowserContext::GetPermissionManager() {
  if (!permission_manager_.get())
    permission_manager_.reset(new AtomPermissionManager);
  return permission_manager_.get();
}
std::unique_ptr<net::CertVerifier> AtomBrowserContext::CreateCertVerifier() {
  return base::WrapUnique(new AtomCertVerifier(ct_delegate_.get()));
}
net::SSLConfigService* AtomBrowserContext::CreateSSLConfigService() {
  return new AtomSSLConfigService;
}
std::vector<std::string> AtomBrowserContext::GetCookieableSchemes() {
  auto default_schemes = brightray::BrowserContext::GetCookieableSchemes();
  const auto& standard_schemes = atom::api::GetStandardSchemes();
  default_schemes.insert(default_schemes.end(),
                         standard_schemes.begin(), standard_schemes.end());
  return default_schemes;
}
net::TransportSecurityState::RequireCTDelegate*
AtomBrowserContext::GetRequireCTDelegate() {
  return ct_delegate_.get();
}
void AtomBrowserContext::RegisterPrefs(PrefRegistrySimple* pref_registry) {