Reland "Remove the 'gyp_config' concept from MB."
This re-lands #349043 with some fixes for win32; we weren't consistently
using os.sep everywhere. This patch fixes that and consolidates the code
so that all references to os.sep and os.path.join() happen in the same
places. In addition, this patch updates the unittests to run cleanly on
win32 and test a win32-specific example (to catch the case that broke).
TBR=scottmg@chromium.org
BUG=481692
Review URL: https://codereview.chromium.org/
1348153003
Cr-Commit-Position: refs/heads/master@{#349477}