* 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
pref_registry->RegisterFilePathPref(prefs::kSelectFileLastDirectory,
                                      base::FilePath());
  base::FilePath download_dir;
  PathService::Get(chrome::DIR_DEFAULT_DOWNLOADS, &download_dir);
  pref_registry->RegisterFilePathPref(prefs::kDownloadDefaultDirectory,
                                      download_dir);
  pref_registry->RegisterDictionaryPref(prefs::kDevToolsFileSystemPaths);
}
AtomBlobReader* AtomBrowserContext::GetBlobReader() {
  if (!blob_reader_.get()) {
    content::ChromeBlobStorageContext* blob_context =
        content::ChromeBlobStorageContext::GetFor(this);
    storage::FileSystemContext* file_system_context =
        content::BrowserContext::GetStoragePartition(
            this, nullptr)->GetFileSystemContext();
    blob_reader_.reset(new AtomBlobReader(blob_context,
                                          file_system_context));
  }
  return blob_reader_.get();
}
scoped_refptr<AtomBrowserContext> AtomBrowserContext::From(
    const std::string& partition, bool in_memory,
    const base::DictionaryValue& options) {
  auto browser_context = brightray::BrowserContext::Get(partition, in_memory);
  if (browser_context)
    return static_cast<AtomBrowserContext*>(browser_context.get());
  return new AtomBrowserContext(partition, in_memory, options);
}
}