url: Properly disable 4267 warning on Windows.

BUG=None
TEST=gn gen out/Debug_gn --args='os="win"'
TBR=scottmg@chromium.org,brettw@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/324633002

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@277033 4ff67af0-8c30-449e-8e8b-ad334ec8d88c