Replace CHROMEOS macro with OS_CHROMEOS

CHROMEOS macro is defined in third_party/webrtc.
This CL replaces it with more appropriate macro.

Bug: None
Change-Id: Ia62a2a1e4b129f7aef7aadb2e7d8158ff06b22f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022045Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735660}

chrome/browser/download/download_service_factory.cc

@@ -59,7 +59,7 @@ std::unique_ptr<download::Client> CreateBackgroundFetchDownloadClient(
   return std::make_unique<BackgroundFetchDownloadClient>(profile);
 }
 
-#if defined(CHROMEOS)
+#if defined(OS_CHROMEOS)
 std::unique_ptr<download::Client> CreatePluginVmImageDownloadClient(
     Profile* profile) {
   return std::make_unique<plugin_vm::PluginVmImageDownloadClient>(profile);
@@ -139,7 +139,7 @@ std::unique_ptr<KeyedService> DownloadServiceFactory::BuildServiceInstanceFor(
           download::DownloadClient::BACKGROUND_FETCH,
           base::BindOnce(&CreateBackgroundFetchDownloadClient), key)));
 
-#if defined(CHROMEOS)
+#if defined(OS_CHROMEOS)
   if (!key->IsOffTheRecord()) {
     clients->insert(std::make_pair(
         download::DownloadClient::PLUGIN_VM_IMAGE,