require c++14 public headers

It occurred to me that we try to guarantee this but don't test it.
Luckily this public_headers_warnings_check target is a perfect spot.

Change-Id: Ie06732706ee80da798a9c3bda215d5a65f02b5bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295856
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 6edbb82..0aad6b3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1266,10 +1266,21 @@
     include_dirs = [ "include/third_party/vulkan" ]
   }
 
+  config("cpp14") {
+    if (is_win) {
+      cflags_cc = [ "/std:c++14" ]
+    } else {
+      cflags_cc = [ "-std=c++14" ]
+    }
+  }
+
   source_set("public_headers_warnings_check") {
     sources = [ "tools/public_headers_warnings_check.cpp" ]
     configs -= [ "//gn:warnings_except_public_headers" ]
-    configs += [ ":our_vulkan_headers" ]
+    configs += [
+      ":our_vulkan_headers",
+      ":cpp14",
+    ]
     deps = [
       ":skia",
       ":skia.h",