* 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
bool in_memory,
                                       const base::DictionaryValue& options)
    : brightray::BrowserContext(partition, in_memory),
      ct_delegate_(new AtomCTDelegate),
      network_delegate_(new AtomNetworkDelegate),
      cookie_delegate_(new AtomCookieDelegate) {
  
  Browser* browser = Browser::Get();
  std::string name = RemoveWhitespace(browser->GetName());
  std::string user_agent;
  if (name == ATOM_PRODUCT_NAME) {
    user_agent = "Chrome/" CHROME_VERSION_STRING " "
                 ATOM_PRODUCT_NAME "/" ATOM_VERSION_STRING;
  } else {
    user_agent = base::StringPrintf(
        "%s/%s Chrome/%s " ATOM_PRODUCT_NAME "/" ATOM_VERSION_STRING,
        name.c_str(),
        browser->GetVersion().c_str(),
        CHROME_VERSION_STRING);
  }
  user_agent_ = content::BuildUserAgentFromProduct(user_agent);
  
  use_cache_ = true;
  options.GetBoolean("cache", &use_cache_);
  
  InitPrefs();
}
AtomBrowserContext::~AtomBrowserContext() {