Add iOS Metal Build job.

Bug: skia:8230
Change-Id: Ic8c17954c5dcaeebcfd028e24a0556d0b09a5b62
Reviewed-on: https://skia-review.googlesource.com/146168
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 3872bc4..fb64695 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -632,6 +632,7 @@
     public_defines += [ "SK_METAL" ]
     sources += skia_metal_sources
     libs += [ "Metal.framework" ]
+    libs += [ "Foundation.framework" ]
     cflags_objcc += [ "-fobjc-arc" ]
   }