Clean up deprecated sources assignment filter in //components/ntp_tiles

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

Bug: 1018739
Change-Id: Ie67a6515da3e08441081fe6c0d1aedcf090366d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445477
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814594}

components/ntp_tiles/BUILD.gn

@@ -6,19 +6,10 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-# 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)
-
 static_library("ntp_tiles") {
   sources = [
     "constants.cc",
     "constants.h",
-    "country_code_ios.h",
-    "country_code_ios.mm",
     "custom_links_manager.h",
     "custom_links_manager_impl.cc",
     "custom_links_manager_impl.h",
@@ -78,6 +69,13 @@ static_library("ntp_tiles") {
     "//services/network/public/cpp",
     "//ui/base",
   ]
+
+  if (is_ios) {
+    sources += [
+      "country_code_ios.h",
+      "country_code_ios.mm",
+    ]
+  }
 }
 
 source_set("unit_tests") {