In e2efb5d4, I fixed shared_library builds on Windows, but I
introduced another issue in which the libpackager_type variable was
not correctly defined by default. This meant that the build only
worked with this variable explicitly-defined in GYP_DEFINES when
gclient sync was run.
This fixes the default definition so that libpackager_type does not
need to be defined explicity.
Related to #318 (shared_library builds on Windows)
Issue #336 (progress toward GitHub Actions workflow to replace Travis
and Appveyor, where we need to build and test shared_library on all
platforms)
b/190743862 (internal; tracking replacement of Travis)
Change-Id: If353e1d3c312ab0c568d4d4d2b789e922d7216e1