Create intermediate PDB files in sub directories.

This patch does not fix any problem actually but makes the build results
cleaner. With this patch, only final PDB files such as Chrome.exe.pdb will be
created in out/Debug or out/Release. All other intermediate PDB files for static libraries will be
created under out/Debug/obj or out/Release/obj alongside other object files.
Review URL: https://codereview.chromium.org/11697006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1558 78cadc50-ecff-11dd-a971-7dbc132099af