Revert "Stop the progress spinner when the main document is complete."

This reverts r173905. I believe it's causing browser_test
and content_browsertest failures, e.g.
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/36661.

TBR=pmeenan@chromium.org,japhet@chromium.org

Review URL: https://codereview.chromium.org/280423003

git-svn-id: svn://svn.chromium.org/blink/trunk@173924 bbb929c8-8fbe-4397-9dbb-9b2b20218538