Clean up deprecated sources assignment filter in //testing

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

Bug: 1018739
Change-Id: Ided224b9b9eb717dd63589500816322af1574e23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443014Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813145}

testing/gtest/BUILD.gn

@@ -9,13 +9,6 @@ if (is_ios) {
   import("//build/config/ios/ios_sdk.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)
-
 config("gtest_direct_config") {
   visibility = [ ":*" ]
   defines = [ "UNIT_TEST" ]
@@ -63,9 +56,6 @@ static_library("gtest") {
   }
 
   if (is_apple && gtest_include_objc_support) {
-    if (is_ios) {
-      set_sources_assignment_filter([])
-    }
     sources += [
       "../gtest_mac.h",
       "../gtest_mac.mm",
@@ -73,7 +63,6 @@ static_library("gtest") {
     if (gtest_include_platform_test) {
       sources += [ "../platform_test_mac.mm" ]
     }
-    set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
   }
 
   if (is_ios && gtest_include_ios_coverage) {