Stop //services/network from using global filter for sources

set_sources_assignment_filter() is considered as a mis-feature
of gn (as it makes it difficult to reason about the BUILD.gn
files individually).

Change code in //services/network to check the platform before
adding plaform-specific source files to "sources" variable and
clear the filter to prevent regressions.

Bug: 1018739
Change-Id: I37d58eaef7570af3a5af40a2e36cff87a3db54fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416076
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807984}

services/network/BUILD.gn

@@ -67,8 +67,6 @@ component("network_service") {
     "network_qualities_pref_delegate.h",
     "network_quality_estimator_manager.cc",
     "network_quality_estimator_manager.h",
-    "network_sandbox_hook_linux.cc",
-    "network_sandbox_hook_linux.h",
     "network_service.cc",
     "network_service.h",
     "network_service_network_delegate.cc",
@@ -213,6 +211,13 @@ component("network_service") {
     ]
   }
 
+  if (is_linux || is_chromeos) {
+    sources += [
+      "network_sandbox_hook_linux.cc",
+      "network_sandbox_hook_linux.h",
+    ]
+  }
+
   configs += [ "//build/config/compiler:wexit_time_destructors" ]
 
   deps = [