Require PLATFORM_FILE_BACKUP_SEMANTICS to open a directory

BUG=352429
R=cpu,brettw
TBR=brettw
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/199843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256975 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256975