Don't stop the documentLoader on navigations.

If the new navigation will be canceled by a Browser-side throttle we
don't want to cancel the currently loading document.

BUG=308257, 378716

Review URL: https://codereview.chromium.org/181493007

git-svn-id: svn://svn.chromium.org/blink/trunk@175269 bbb929c8-8fbe-4397-9dbb-9b2b20218538