Don't link against MetalKit when building the base Skia library.

MetalKit was introduced in iOS 9 and contains utilities for building
applications with Metal. None of these utilities actually seem to be in use.
However, linking against MetalKit means Flutter cannot deploy a unified binary
to iOS 8. This patch removes the unused dependency and unblocks Flutter.

Fixes flutter/flutter#64017

Change-Id: I2cf54a1c95b4c3cda96300861598f2cb23f583d6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314539
Reviewed-by: Jim Van Verth <[email protected]>
Reviewed-by: Xiao Yu <[email protected]>
Commit-Queue: Jim Van Verth <[email protected]>

Author: chinmaygarde

BUILD.gn

@@ -832,7 +832,6 @@ optional("gpu") {
     public_defines += [ "SK_METAL" ]
     sources += skia_metal_sources
     frameworks += [ "Metal.framework" ]
-    frameworks += [ "MetalKit.framework" ]
     frameworks += [ "Foundation.framework" ]
     cflags_objcc += [ "-fobjc-arc" ]
   }
@@ -1798,9 +1797,11 @@ if (skia_enable_tools) {
   import("gn/tests.gni")
   test_lib("tests") {
     sources = tests_sources + pathops_tests_sources
+    frameworks = []
     if (skia_use_metal) {
       sources += metal_tests_sources
       cflags_objcc = [ "-fobjc-arc" ]
+      frameworks += [ "MetalKit.framework" ]
     }
     if (skia_use_gl) {
       sources += gl_tests_sources