Bug 1099430, part 一: Define MOZILLA_DIR early in the base makefiles, r=glandium.

This change allows us to use use MOZILLA_DIR even for changes in config.mk. The
corollary is that comm-central needs to define MOZILLA_DIR before calling into
baseconfig.mk, which it already does.

The change to the test Makefile is needed since it decides to skip including
config.mk, and hence baseconfig.mk, where it would need to grab the MOZILLA_DIR
declaration.
Bug 1099430, part 二: Define WIN_TOP_SRC using MOZILLA_DIR, r=glandium.

win_srcdir is used as the Windows path for the srcdir, and so it needs to use
comm-central's topsrcdir to work properly. In contrast, WIN_TOP_SRC is really
only used to define BUILD_TOOLS, which refers to files in mozilla-central/build,
and so that can and should remain the mozilla-central topsrcdir.
Bug 1099430, part 三: Use MOZILLA_DIR instead of topsrcdir in shared makefiles, r=glandium.

With these substitutions, it is possible to have comm-central's versions of
these files merely include the mozilla-central versions and avoid having to port
small changes to these files.