Add Skia/Metal to the Chromium build

Keep it behind a skia_use_metal flag for the moment.

Put the Metal sources in a separate place from skia_util_mac.mm, because
Metal uses ARC and skia_util_mac.mm uses explicit reference counting.

Bug: 952063
Change-Id: I601b3c9814b4beaf6a091871ba4dc2592c2fc498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565387Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652229}

skia/BUILD.gn

@@ -25,6 +25,7 @@ skia_support_skottie = true
 
 declare_args() {
   skia_whitelist_serialized_typefaces = false
+  skia_use_metal = false
 }
 
 # External-facing config for dependent code.
@@ -555,6 +556,15 @@ component("skia") {
   }
 
   if (is_mac) {
+    if (skia_use_metal) {
+      deps += [ ":skia_metal" ]
+      defines += [ "SK_METAL" ]
+      libs += [
+        "Metal.framework",
+        "Foundation.framework",
+      ]
+    }
+
     libs = [
       "AppKit.framework",
       "CoreFoundation.framework",
@@ -790,6 +800,18 @@ skia_source_set("skia_opts") {
   visibility = [ ":skia" ]
 }
 
+# Split out metal sources, because they require ARC.
+if (is_mac && skia_use_metal) {
+  skia_source_set("skia_metal") {
+    defines = [ "SK_METAL" ]
+    sources = skia_metal_sources
+    cflags_objcc = [
+      "-Wno-unguarded-availability",
+      "-fobjc-arc",
+    ]
+  }
+}
+
 # Font copies.
 if (is_android || is_fuchsia) {
   copy("copy_android_fonts_config") {