[ios] Enable New Build System in generated Xcode project

Since gn now generates a project that wraps the invocation
of ninja in a script converting relative paths to absolute
paths, there is nothing blocking the use of the New Build
System.

Bug: 852522
Change-Id: I9b4ca343dc9434efb64a3866ca88d6b7c4065760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276281Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785343}

ios/build/tools/setup-gn.py

@@ -209,6 +209,7 @@ class GnGenerator(object):
     if generate_xcode_project:
       gn_command.append('--ide=xcode')
       gn_command.append('--root-target=gn_all')
+      gn_command.append('--xcode-build-system=new')
       gn_command.append('--ninja-executable=autoninja')
       if self._settings.has_section('filters'):
         target_filters = self._settings.values('filters')