Compile Skia .mm files with C++17 support

Change-Id: Ied5a58752c15cee95ba053e39e1c31e06d09f361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937649
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/master@{#719342}

skia/BUILD.gn

@@ -199,6 +199,9 @@ config("skia_library_config") {
   } else {
     cflags_cc = [ "-std=c++17" ]
   }
+  if (is_mac) {
+    cflags_objcc = [ "-std=c++17" ]
+  }
 }
 
 source_set("skcms") {
@@ -808,6 +811,7 @@ if (is_mac) {
     cflags_objcc = [
       "-Wno-unguarded-availability",
       "-fobjc-arc",
+      "-std=c++17",
     ]
     deps = [
       "//gpu/config:workaround_list",