Native Client side changes for debugging support: stdout/err -> postmessage.

Native Client side changes for debugging support: stdout/err -> postmessage.

This CL contains NaCl-side changes to allow NACL_EXE_STDOUT and
NACL_EXE_STDERR environment variables to be set to
"DEBUG_ONLY:dev://postmessage" to have output to those descriptors be
sent (via the reverse channel) to the plugin.  Once there, Chrome-side
changes (in a separate Cr-side CL) will handle the CallOnMainThread
etc to PostMessage the data for the JavaScript code to process, e.g.,
console.log the message.

TEST= run_pm_redir_{stdout,stderr}_{fg,bg}_{0,1000,1000000}_chrome_browser_test
BUG= http://code.google.com/p/nativeclient/issues/detail?id=2474