Pepper: Clean up PpapiDispatcher parent classes.

This change modifies PpapiDispatcher to inherit IPC::Listener and IPC::Sender
instead of ProxyChannel directly. This cleans up the class hierarchy a bit.

BUG=
R=dmichael@chromium.org

Review URL: https://codereview.chromium.org/172103008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252108 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252108