Default msvs_cygwin_shell to 0

This will cause all rules that do not have msvs_cygwin_shell: 1 to run
using cmd.exe.

R=iannucci@chromium.org
TBR=wez@chromium.org
BUG=123026

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=237623

Review URL: https://codereview.chromium.org/86703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237636 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237636