Opening a named pipe can block until both ends are open, and we cannot
control when the other end will be open. Ideally, we would always
open files in a thread so that Packager can be used with piped inputs
from naive applications without a potential deadlock.
This change will defer opening WebVTT files until the parser Run()
method is called from a thread. This way, WebVTT files being sent in
from a pipe will never be able to block the main thread.
Previously, files were opened on the main thread before calling the
parser constructor, passing the open file to the constructor as an
argument. I also tried doing it in the parser's InitializeInternal()
method, but that is also called from the main thread.
Change-Id: I54cc68ed9d48a8dc697829119be84d4065b1ae1c
82bc7a3b95