net: FileStream cleanup
Remove old "Async" prefix from a time when FileStream could perform sync
operations, and use base::File methods instead of reimplement them.
Remove a TODO from native_message_process_host given that we decided it is
not worth moving that functionality to FileStream ATM.
BUG=322664
TBR=sergeyu@chromium.org, kinuko@chromium.org
R=hashimoto@chromium.org
Review URL: https://codereview.chromium.org/
323683002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278857 0039d316-1c4b-4281-b951-d872f2087c98