Fix "defined but not used" warning in ppapi/proxy/ppb_instance_proxy.cc

PNaCl is being upgraded to Clang 3.4, which now warns about unused
declarations inside anonymous namespaces.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3757
TEST=compile using new PNaCl toolchain
R=teravest@chromium.org

Review URL: https://codereview.chromium.org/185293010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254530 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254530