[cros] Better handling of network which requires proxy on login screen.

Several minor issues were fixed:
1. Avoid obsessive showing of captive portal overlay window.
2. Reload login extension, when proxy setting window is closed (fixed bug).
3. Do not cover ProxySettingDialog with captive portal overaly.
4. Extended comments coverage.

BUG=chromium:121763
TEST=manual

Review URL: https://chromiumcodereview.appspot.com/9978008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@130749 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/chromeos/login/captive_portal_window_proxy.cc[diff]
• chrome/browser/chromeos/login/proxy_settings_dialog.cc[diff]
• chrome/browser/chromeos/login/proxy_settings_dialog.h[diff]
• chrome/browser/resources/chromeos/login/screen_error_message.js[diff]