Mask the introspection USE flag at profile level.

chromeos/config/make.conf.common-target

 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # NOTE: This assumes that the following have already been defined:
 #  { ROOT, ARCH, CHOST, [BOARD_OVERLAY] }
 
 # TODO: This will have to come from somewhere else when we support a 32-bit
 # build host environment.
 CBUILD=x86_64-pc-linux-gnu
 HOSTCC=x86_64-pc-linux-gnu-gcc
 
 ACCEPT_KEYWORDS="${ARCH}"
-USE="${ARCH} zlib bindist make-symlinks cros-debug"
+USE="${ARCH} zlib bindist make-symlinks cros-debug -introspection"
 
 # Be sure we don't overwrite pkgs from another sysroot.
 PKGDIR=${ROOT}packages/
 PORTAGE_TMPDIR=${ROOT}tmp/
 
 FEATURES="buildpkg nodoc noinfo noman sandbox splitdebug -strict userfetch \
           userpriv usersandbox"
 
 PORTAGE_WORKDIR_MODE="0755"
 PKG_CONFIG_PATH="${ROOT}usr/lib/pkgconfig/:${ROOT}usr/share/pkgconfig/"
@@ skipping 32 matching lines @@
 SHARED_USER_HOME="/home/chronos/user"
 
 # Packages that use python will run a small python script to find the
 # pythondir. Unfortunately, if the build chroot is 64-bit then that
 # will output /usr/lib64/python2.6 rather than /usr/lib/python2.6.
 # TODO(tedbo): Find a better place or fix for this "hack". 
 am_cv_python_pythondir="\${prefix}/lib/python2.6/site-packages"
 
 # Allow the user to override or define additional settings.
 source make.conf.user