Files.app: Remove maybeScheduleCloseBackgroundPage_ from FileOperationManager.

This check should be in Background class.  This CL updates the handler of file
operation manager to request to close the Background class.

BUG=325086
TEST=close Files.app, see background page in the inspector.

Review URL: https://codereview.chromium.org/107883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239458 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239458