Pepper: Remove unused field in message.

PpapiHostMsg_CreateNaClChannel has a field for the renderer_id, but it's never
used.

I came across this when figuring out how to create two channels between the
renderer and plugin, one solely for communication with the "trusted" part of
the plugin.

BUG=
R=bbudge@chromium.org, dmichael@chromium.org
TBR=jschuh, mseaborn

Review URL: https://codereview.chromium.org/108383006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242893 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242893