Stop the progress spinner when the main document is complete.

Added a call to tell the progress indicator that the load is complete
when the main frame reaches the completed state. This fixes the case
where the spinner keeps spinning until all activity has stopped even
after the onload event has fired.

This is taking the easy way out by not modifying the actual end state
tracked by the loader so it doesn't change any downstream behavior
(like all of the layout tests). It does mean that the progress
indicator will be told that the load is finished twice so that was
also modified to only process the first call but allow multiple calls.

BUG=123334

Review URL: https://codereview.chromium.org/280333002

git-svn-id: svn://svn.chromium.org/blink/trunk@173905 bbb929c8-8fbe-4397-9dbb-9b2b20218538