Add support for creating a GrContext backed by Metal.

Also adds the support code to allow our TestContext to create a Metal
backend.

Bug: skia:
Change-Id: Ia850687019d79b897bb16e2c151f4f8526721ad9
Reviewed-on: https://skia-review.googlesource.com/22644
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 26eeb95..4c34a1e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -994,6 +994,9 @@
           libs += [ "vulkan" ]
         }
       }
+      if (skia_use_metal) {
+        sources += [ "tools/gpu/mtl/MtlTestContext.mm" ]
+      }
     }
   }