Add --disable-background-networking chromium flag

Chromium browser docs describe this as follows:
Disable several subsystems which run network requests in the
background. This is for use when doing network performance testing to
avoid noise in the measurements.

Testing indicates that irrelevant HTTP requests like the following stop
with this imporvement.
```
HEAD http://ugfgntuqva/ HTTP/1.1
```

brozzler/chrome.py

@@ -126,6 +126,7 @@ class Chrome:
                 '--remote-debugging-port=%s' % self.port,
                 '--use-mock-keychain', # mac thing
                 '--user-data-dir=%s' % self._chrome_user_data_dir,
+                '--disable-background-networking',
                 '--disable-web-sockets', '--disable-cache',
                 '--window-size=1100,900', '--no-default-browser-check',
                 '--disable-first-run-ui', '--no-first-run',