Add a default value to num_processor in case multithreaded doesn't exist on windows.
Add a default value to num_processor in case multithreaded doesn't exist on windows.

TBR=maruel@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11852030

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@176418 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py