Use relative path/dir msbuild variables to map $(InputDir) and $(InputPath)

$(InputDir) and $(InputPath) assume relative paths, but they're mapped to
%(RootDir)%(Directory) and %(FullPath) respectively. This makes gyp
generate incorrect commands for non-external rules in VS, for example:

-I ../foo/C:/bar/baz.h

(based on a generated command when building content_shell in VS)

MSBuild also provides variables for relative dir and path, so we should
use those instead to map $(InputDir) and $(InputPath).

Ran test/msvs and test/win test suites against VS2010 and VS2013, no regression.
Since VS maintains the cwd when building, using relative paths shouldn't affect
the current behavior. The only case where it might break things is if an
external script/command expects absolute paths for its arguments. I haven't
seen any instance of this while building content_shell and its dependencies.

BUG=

Patch from romain.pokrzywka@gmail.com.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/294183004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1924 78cadc50-ecff-11dd-a971-7dbc132099af