GN: add public headers warnings check.

We already generate skia.h to include all public headers for Fiddle.
This just includes it with -Wunused-parameter turned on as an error.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4901

Change-Id: Ia55a901c09a3c9c9d6d35a43259431dba3532ed9
Reviewed-on: https://skia-review.googlesource.com/4901
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

BUILD.gn

@@ -686,7 +686,11 @@ component("skia") {
   }
 }
 
+config("skia.h_config") {
+  include_dirs = [ "$target_gen_dir" ]
+}
 action("skia.h") {
+  public_configs = [ ":skia.h_config" ]
   skia_h = "$target_gen_dir/skia.h"
   script = "gn/find_headers.py"
   args = [ rebase_path(skia_h, root_build_dir) ] +
@@ -701,7 +705,6 @@ if (skia_enable_gpu && target_cpu == "x64") {
   # Our bots only have 64-bit libOSMesa installed.
   # TODO: worth fixing?
   executable("fiddle") {
-    include_dirs = [ "$target_gen_dir" ]
     libs = []
     if (is_linux) {
       libs += [ "OSMesa" ]
@@ -719,6 +722,19 @@ if (skia_enable_gpu && target_cpu == "x64") {
   }
 }
 
+if (is_skia_standalone) {
+  source_set("public_headers_warnings_check") {
+    sources = [
+      "tools/public_headers_warnings_check.cpp",
+    ]
+    configs -= [ "//gn:warnings_except_public_headers" ]
+    deps = [
+      ":skia",
+      ":skia.h",
+    ]
+  }
+}
+
 # Targets guarded by skia_enable_tools may use //third_party freely.
 if (skia_enable_tools) {
   template("test_lib") {

gn/BUILD.gn

@@ -276,7 +276,6 @@ config("warnings") {
       "-Wvla",
 
       "-Wno-deprecated-declarations",
-      "-Wno-unused-parameter",
     ]
     cflags_cc += [ "-Wnon-virtual-dtor" ]
 
@@ -347,6 +346,11 @@ config("warnings") {
     }
   }
 }
+config("warnings_except_public_headers") {
+  if (!is_win) {
+    cflags = [ "-Wno-unused-parameter" ]
+  }
+}
 
 config("extra_flags") {
   cflags = extra_cflags

gn/BUILDCONFIG.gn

@@ -114,6 +114,7 @@ default_configs = [
   "//gn:no_exceptions",
   "//gn:no_rtti",
   "//gn:warnings",
+  "//gn:warnings_except_public_headers",
 ]
 if (!is_debug) {
   default_configs += [ "//gn:release" ]

tools/public_headers_warnings_check.cpp

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "skia.h"