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: I0587d5958963eb38b44372837d8098db774214fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021788Reviewed-by: Brandon Tolsch <btolsch@chromium.org>
Commit-Queue: Hiroki Sato <hirokisato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736176}

chrome/browser/ui/views/media_router/presentation_receiver_window_view.cc

@@ -314,7 +314,7 @@ void PresentationReceiverWindowView::UpdateExclusiveAccessExitBubbleContent(
     ExclusiveAccessBubbleType bubble_type,
     ExclusiveAccessBubbleHideCallback bubble_first_hide_callback,
     bool force_update) {
-#if defined(CHROMEOS)
+#if defined(OS_CHROMEOS)
   // On Chrome OS, we will not show the toast for the normal browser fullscreen
   // mode.  The 'F11' text is confusing since how to access F11 on a Chromebook
   // is not common knowledge and there is also a dedicated fullscreen toggle

chrome/browser/ui/views/media_router/presentation_receiver_window_view_browsertest.cc

@@ -25,7 +25,7 @@
 #include "ui/views/view.h"
 #include "url/gurl.h"
 
-#if defined(CHROMEOS)
+#if defined(OS_CHROMEOS)
 #include "ash/public/cpp/window_properties.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/window.h"