Clean up deprecated sources assignment filter in //third_party/libxslt

Remove deprecated_default_sources_assignment_filter and replace
it with explicit is_win, is_mac, etc. sections.

Bug: 1018739
Change-Id: Icfd0d38fc491303aaec2a9249843c117e164c005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440845Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813142}

third_party/libxslt/BUILD.gn

@@ -2,13 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# This file depends on the legacy global sources assignment filter. It should
-# be converted to check target platform before assigning source files to the
-# sources variable. Remove this import and set_sources_assignment_filter call
-# when the file has been converted. See https://crbug.com/1018739 for details.
-import("//build/config/deprecated_default_sources_assignment_filter.gni")
-set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
-
 config("libxslt_config") {
   defines = [ "LIBXSLT_STATIC" ]
   include_dirs = [ "src" ]
@@ -29,8 +22,6 @@ config("libxslt_warnings") {
 
 static_library("libxslt") {
   sources = [
-    "linux/config.h",
-    "mac/config.h",
     "src/libxslt/attributes.c",
     "src/libxslt/attributes.h",
     "src/libxslt/attrvt.c",
@@ -87,6 +78,13 @@ static_library("libxslt") {
   ]
   public_configs = [ ":libxslt_config" ]
 
+  if (is_linux || is_chromeos) {
+    sources += [ "linux/config.h" ]
+  }
+  if (is_mac) {
+    sources += [ "mac/config.h" ]
+  }
+
   cflags = []
   if (is_linux || is_chromeos || is_android || is_fuchsia) {
     include_dirs = [ "linux" ]