Canonicalize toolpath in build_nexe.
Canonicalize toolpath in build_nexe.

Ninja on windows is failing when invoking the pnacl toolchain,
due to a mix of forward and backward slashes.
Canonicalizing the slashes in toolpath to insulate the tools
we invoke.

BUG=http://code.google.com/p/chromium/issues/detail?id=151717
TEST=builds with winja
R=dschuff@chromium.org,scottmg@chromium.org

Review URL: https://codereview.chromium.org/10961062

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@9823 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client