Add Metal context to Viewer.

Bug: skia:8737
Change-Id: I5c4c839bcf39f2cd3a9a32d005bf4bdb7c42e6a5
Reviewed-on: https://skia-review.googlesource.com/c/187925
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index a488e64..96fdf86 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2162,6 +2162,14 @@
       }
     }
 
+    if (skia_use_metal) {
+      sources += [ "tools/sk_app/MetalWindowContext.mm" ]
+      if (is_mac) {
+        sources += [ "tools/sk_app/mac/MetalWindowContext_mac.mm" ]
+      }
+      libs += [ "MetalKit.framework" ]
+    }
+
     deps = [
       ":gpu_tool_utils",
       ":skia",