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

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

Bug: 1018739
Change-Id: If84d88a29e4dc9602494d16446b6b85eefc35149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445476Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813681}

components/gwp_asan/client/BUILD.gn

@@ -4,25 +4,21 @@
 
 import("//base/allocator/allocator.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)
-
 component("client") {
   output_name = "gwp_asan_client"
   sources = [
     "export.h",
     "guarded_page_allocator.cc",
     "guarded_page_allocator.h",
-    "guarded_page_allocator_win.cc",
     "gwp_asan.cc",
     "gwp_asan.h",
     "sampling_state.h",
   ]
 
+  if (is_win) {
+    sources += [ "guarded_page_allocator_win.cc" ]
+  }
+
   if (is_posix) {
     sources += [ "guarded_page_allocator_posix.cc" ]
   }