Add c++17 for objective-c++ for Skia

The Mac Metal code uses objective-c++, but was still using the c++14
command-line flag.

Change-Id: I150ad2369326e558cad966714a6a966ba7c77dba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937657
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#719356}

skia/BUILD.gn

@@ -198,6 +198,7 @@ config("skia_library_config") {
     cflags_cc = [ "/std:c++17" ]
   } else {
     cflags_cc = [ "-std=c++17" ]
+    cflags_objcc = [ "-std=c++17" ]
   }
 }