Fix <rdar://problem/5432686> 333MB RPRVT seems to leak @ www.43folders.com (1hr plug...

Fix <rdar://problem/5432686> 333MB RPRVT seems to leak @ 43folders.com (1hr plug-in stream).
http://bugs.webkit.org/show_bug.cgi?id=13705

Reviewed by Tim Hatcher.

Have NP_ASFILE and NP_ASFILEONLY streams write the data to disk as they receive it rather than
dumping the data to disk in a single go when the stream has completed loading.  On a test case
involving a 150MB Flash movie being streamed from a local web server this reduces memory consumption
on page load from around 400MB to 22MB.

The only plugin I have found that uses NP_ASFILE or NP_ASFILEONLY on the Mac is our NetscapeMoviePlugin
example code so the NP_ASFILE portion of this change has not had any testing with a real-world plugin.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@28006 268f45cc-cd09-0410-ab3c-d52691b4dbfc