To see the main effect of this, look at the doxygen for file
IceGlobalContext.h to see the effect of turning this on.
Without it, the nested classes are not available because they
are private.

My guess is that often doxygen is used to generate API documents and for that reason, exporting of private info is excluded by default.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1411123019 .