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

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

Bug: 1018739
Change-Id: Ib8f8d20263a3d51627f00a47261adae1b519bd38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446150
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814356}

components/history/core/test/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)
-
 static_library("test") {
   testonly = true
   sources = [
@@ -27,9 +20,7 @@ static_library("test") {
     "test_history_database.cc",
     "test_history_database.h",
     "thumbnail-inl.h",
-    "thumbnail.cc",
     "thumbnail.h",
-    "thumbnail_ios.mm",
     "wait_top_sites_loaded_observer.cc",
     "wait_top_sites_loaded_observer.h",
   ]
@@ -49,6 +40,8 @@ static_library("test") {
   ]
 
   if (is_ios) {
-    sources -= [ "thumbnail.cc" ]
+    sources += [ "thumbnail_ios.mm" ]
+  } else {
+    sources += [ "thumbnail.cc" ]
   }
 }