Canonicalize output directory argument in GN

On Windows we would previously get confused if you used blackslashes in the output dir name. This just converts them to forward slashes.

BUG=

Review URL: https://codereview.chromium.org/66653004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234440 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234440