* 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
return cookie_delegate();
}
std::string AtomBrowserContext::GetUserAgent() {
  return user_agent_;
}
std::unique_ptr<net::URLRequestJobFactory>
AtomBrowserContext::CreateURLRequestJobFactory(
    content::ProtocolHandlerMap* protocol_handlers) {
  std::unique_ptr<AtomURLRequestJobFactory> job_factory(
      new AtomURLRequestJobFactory);
  for (auto& it : *protocol_handlers) {
    job_factory->SetProtocolHandler(it.first,
                                    base::WrapUnique(it.second.release()));
  }
  protocol_handlers->clear();
  job_factory->SetProtocolHandler(
      url::kDataScheme, base::WrapUnique(new net::DataProtocolHandler));
  job_factory->SetProtocolHandler(
      url::kFileScheme, base::WrapUnique(new asar::AsarProtocolHandler(
          BrowserThread::GetBlockingPool()->GetTaskRunnerWithShutdownBehavior(
              base::SequencedWorkerPool::SKIP_ON_SHUTDOWN))));
  job_factory->SetProtocolHandler(
      url::kHttpScheme,
      base::WrapUnique(new HttpProtocolHandler(url::kHttpScheme)));
  job_factory->SetProtocolHandler(
      url::kHttpsScheme,
      base::WrapUnique(new HttpProtocolHandler(url::kHttpsScheme)));
  job_factory->SetProtocolHandler(
      url::kWsScheme,
      base::WrapUnique(new HttpProtocolHandler(url::kWsScheme)));