win: make msvs_shard work properly for static_library references

Before this, dependencies inside a static_library weren't updated to
reference the _N targets, necessitating workarounds like
https://src.chromium.org/viewvc/chrome?revision=254345&view=revision .

The 'refs_to_shard_external_lib' target fails before this change.

R=thakis@chromium.org
BUG=chromium:352495

Review URL: https://codereview.chromium.org/199413010

git-svn-id: http://gyp.googlecode.com/svn/trunk@1873 78cadc50-ecff-11dd-a971-7dbc132099af