Compile GURL for PNaCl

The new url_nacl target compiles GURL for PNaCl.

BUG=276739

Review URL: https://codereview.chromium.org/174553004

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@252886 4ff67af0-8c30-449e-8e8b-ad334ec8d88c