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: svn://svn.chromium.org/chrome/trunk/src@252886 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252886