Update Metal backend to account for Skia updates. (#9383)

Also makes it so that folks don't have to patch the buildroot manually when
building for Metal.

DEPS

@@ -122,7 +122,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'd1bbc1496ab1e239169f1dcb29e706e76c46ddaa',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '7a3ec7d15ccd753a270a714c3972cef4e8660ffb',
 
    # Fuchsia compatibility
    #

shell/gpu/gpu_surface_metal.mm

@@ -8,6 +8,7 @@
 
 #include "third_party/skia/include/core/SkSurface.h"
 #include "third_party/skia/include/gpu/GrBackendSurface.h"
+#include "third_party/skia/include/ports/SkCFObject.h"
 
 namespace flutter {
 
@@ -80,7 +81,7 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetal::AcquireFrame(const SkISize& size)
   }
 
   GrMtlTextureInfo metal_texture_info;
-  metal_texture_info.fTexture = metal_texture.get();
+  metal_texture_info.fTexture.reset(SkCFSafeRetain(metal_texture.get()));
 
   GrBackendRenderTarget metal_render_target(bounds.width * scale,   // width
                                             bounds.height * scale,  // height

tools/gn

@@ -229,6 +229,7 @@ def to_gn_args(args):
       gn_args['skia_use_metal'] = True
       gn_args['shell_enable_metal'] = True
       gn_args['ios_deployment_target'] = '11'
+      gn_args['allow_deprecated_api_calls'] = True
 
     if args.enable_vulkan:
       # Enable vulkan in the Flutter shell.