Get Metal running on iOS viewer.

Adds MetalWindowContext_ios and hooks it up to Window_ios.
Also includes some minor clean up in other iOS code.

Bug: skia:8737
Change-Id: I2e8a0c755310fbc4ed534f975815c60f8eca130b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238438
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index a36d610..5a53a94 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2421,7 +2421,7 @@
       if (is_mac) {
         sources += [ "tools/sk_app/mac/MetalWindowContext_mac.mm" ]
       } else if (is_ios) {
-        #        sources += [ "tools/sk_app/mac/MetalWindowContext_ios.mm" ]
+        sources += [ "tools/sk_app/ios/MetalWindowContext_ios.mm" ]
       }
     }