Clean up deprecated sources assignment filter in //content

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

Bug: 1018739
Change-Id: I3befa68bf2f1091392fa46dec43660638189f637
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443250
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813210}

content/BUILD.gn

@@ -6,13 +6,6 @@ import("//build/config/nacl/config.gni")
 import("//ppapi/buildflags/buildflags.gni")
 import("//tools/grit/grit_rule.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)
-
 # Applied by targets internal to content.
 config("content_implementation") {
   defines = [ "CONTENT_IMPLEMENTATION" ]
@@ -93,7 +86,6 @@ if (is_component_build) {
   }
 } else if (is_nacl_nonsfi) {
   source_set("content") {
-    set_sources_assignment_filter([])
     sources = [
       "//sandbox/policy/linux/sandbox_seccomp_bpf_linux.cc",
       "common/sandbox_init_linux.cc",
@@ -101,7 +93,6 @@ if (is_component_build) {
       "public/common/content_switches.cc",
       "public/common/content_switches.h",
     ]
-    set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
     deps = [
       "//base",
       "//media:media_buildflags",