[ios] Fix handling of empty string in ~/setup-gn

If a variable was initialised to an empty string, it would be
generated as `variable = ` instead of `variable = ""` when the
args.gn file was generated, breaking `gn gen`.

Convert the string to the correct format.

Bug: none
Change-Id: I6268623c3e6cd2c27bfdfc167a4a77779e01962f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2282566
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786267}

ios/build/tools/setup-gn.py

@@ -168,6 +168,10 @@ class GnGenerator(object):
               stream.write('%s%s%s' % (prefix, item, suffix))
           stream.write(']\n')
         else:
+          # ConfigParser removes quote around empty string which confuse
+          # `gn gen` so restore them.
+          if not value:
+            value = '""'
           stream.write('%s = %s\n' % (name, value))
 
   def WriteBuildNinja(self, gn_command, build_dir):