win: Use the _winreg module for registry access

This commit changes the Visual Studio installation path detection code to use
_winreg to find the installation path in the registry.  The previous method of
using reg.exe is preserved as a fallback for when '_winreg' cannot be imported
(for example in cygwin).

Also, remove the '_RegistryKeyExists' function since it is not used anywhere.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/724863002

git-svn-id: http://gyp.googlecode.com/svn/trunk@2002 78cadc50-ecff-11dd-a971-7dbc132099af
ninja win: Add msvs emulation for FloatingPointModel

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/720153005

git-svn-id: http://gyp.googlecode.com/svn/trunk@2003 78cadc50-ecff-11dd-a971-7dbc132099af