Use FrameStateComplete to determine when to print, not DocumentLoader::isLoading

BUG=372783
TEST=WebViewTest.PrintWithXHRInFlight

Review URL: https://codereview.chromium.org/296293003

git-svn-id: svn://svn.chromium.org/blink/trunk@174878 bbb929c8-8fbe-4397-9dbb-9b2b20218538