NaCl: Move definition of "struct nacl_irt_ppapihook" to the Chromium repo

This will allow the definition to be removed from NaCl's irt.h.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3787
TEST=build
R=teravest@chromium.org

Review URL: https://codereview.chromium.org/172633005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252185 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252185