Add LaunchSelLdr and UrandomFD to PPAPI

LaunchSelLdr just calls through to the webkit_glue backend, which calls through a bunch more layers of abstraction for us.

UrandomFD returns the file descriptor of /dev/urandom (on Posix), which NaCl uses for some purpose by can't get directly because of the sandbox.
Review URL: http://codereview.chromium.org/6023003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69578 0039d316-1c4b-4281-b951-d872f2087c98