Clean up deprecated sources assignment filter in //third_party/grpc

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

Bug: 1018739
Change-Id: I911b1aa65b22ad2b2db8059b39feee9479ab003d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440608Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813156}

third_party/grpc/BUILD.gn

@@ -4,13 +4,6 @@
 
 import("//testing/libfuzzer/fuzzer_test.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("grpc_config") {
   include_dirs = [ "src/include" ]
   defines = [
@@ -129,10 +122,6 @@ if (current_toolchain == host_toolchain) {
 static_library("grpc_core") {
   visibility = [ ":*" ]
 
-  # gRPC has some _linux.cc files that provide no-op implementations when the
-  # platform is not Linux. We need to disable source filter so that they can be
-  # built, otherwise linker will complain for undefined symbols.
-  set_sources_assignment_filter([])
   sources = [
     "src/src/core/ext/filters/census/grpc_context.cc",
     "src/src/core/ext/filters/client_channel/backup_poller.cc",
@@ -504,7 +493,7 @@ static_library("grpc_core") {
     "src/src/core/tsi/transport_security.cc",
     "src/src/core/tsi/transport_security_grpc.cc",
   ]
-  set_sources_assignment_filter(deprecated_default_sources_assignment_filter)
+
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":grpc_config",