* 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
job_factory->SetProtocolHandler(
      url::kWssScheme,
      base::WrapUnique(new HttpProtocolHandler(url::kWssScheme)));
  auto host_resolver =
      url_request_context_getter()->GetURLRequestContext()->host_resolver();
  job_factory->SetProtocolHandler(
      url::kFtpScheme,
      base::WrapUnique(new net::FtpProtocolHandler(
          new net::FtpNetworkLayer(host_resolver))));
  return std::move(job_factory);
}
net::HttpCache::BackendFactory*
AtomBrowserContext::CreateHttpCacheBackendFactory(
    const base::FilePath& base_path) {
  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
  if (!use_cache_ || command_line->HasSwitch(switches::kDisableHttpCache))
    return new NoCacheBackend;
  else
    return brightray::BrowserContext::CreateHttpCacheBackendFactory(base_path);
}
content::DownloadManagerDelegate*
AtomBrowserContext::GetDownloadManagerDelegate() {
  if (!download_manager_delegate_.get()) {
    auto download_manager = content::BrowserContext::GetDownloadManager(this);
    download_manager_delegate_.reset(
        new AtomDownloadManagerDelegate(download_manager));
  }
  return download_manager_delegate_.get();
}
content::BrowserPluginGuestManager* AtomBrowserContext::GetGuestManager() {