Added msvs_enable_winrt to enable creating Windows Store compatible projects for .dlls that are consumed by WinRT applications.

Patch from coopp@microsoft.com.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/607553002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1985 78cadc50-ecff-11dd-a971-7dbc132099af
Add support for MIDL include directories

This adds support for 'midl_include_dirs', which is handled like 'include_dirs'
and 'resource_include_dirs', except this is for the MIDL tool.

One notable difference is that, unlike 'resource_include_dirs', the
'midl_include_dirs' property doesn't default to 'include_dirs'.  This is to
prevent any change in behavior for any existing gyp files that don't set
'midl_include_dirs'.  Also, MIDL include directories are quite a different
concept to C/C++ include directories.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/602073003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1986 78cadc50-ecff-11dd-a971-7dbc132099af