Use GYP instead of NONE as the module class for gyp.

The Chromium makefile generator used to use module class NONE for the
meta-targets in the gyp files; this was changed to GYP in order to make
it more obvious where they come from in the build. Update the build
system's handling of NOTICE files to reflect this change.

Change-Id: Iff7d4487f3024ec83db0c2a896ec0caf8fb18b24