Make sure RTC_SUPPORTS_METAL is set in AppRTCMobile.

Otherwise the metal renderer is not used on supported platforms.

Bug: None
Change-Id: I0da56dec1ec8900b7b251794a071663c4badb1de
Reviewed-on: https://webrtc-review.googlesource.com/c/123786
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26803}
diff --git a/examples/BUILD.gn b/examples/BUILD.gn
index bc27983..31bd4de 100644
--- a/examples/BUILD.gn
+++ b/examples/BUILD.gn
@@ -346,6 +346,8 @@
         "objc/AppRTCMobile/ios/UIImage+ARDUtilities.m",
       ]
 
+      configs += [ "..:common_objc" ]
+
       deps = [
         ":apprtc_common",
         ":apprtc_signaling",