Update ui::CompositorObserver instrumentation

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 14b5366b0b536d2a87ef39228c56b1a75def16e1..d4ee6c81ad7f8ac0303e135ee8041e6b9acf7d4e 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -278,10 +278,8 @@ scoped_refptr<CompositorVSyncManager> Compositor::vsync_manager() const {
 }
 
 void Compositor::AddObserver(CompositorObserver* observer) {
-#if defined(OS_MACOSX)
-  // Debugging instrumentation for crbug.com/401630.
-  // TODO(ccameron): remove this.
-  CHECK(observer);
+#if DCHECK_IS_ON
+  DCHECK(observer);
   if (!observer_list_.HasObserver(observer))
     observer->observing_count_ += 1;
 #endif
@@ -290,9 +288,7 @@ void Compositor::AddObserver(CompositorObserver* observer) {
 }
 
 void Compositor::RemoveObserver(CompositorObserver* observer) {
-#if defined(OS_MACOSX)
-  // Debugging instrumentation for crbug.com/401630.
-  // TODO(ccameron): remove this.
+#if DCHECK_IS_ON
   if (observer_list_.HasObserver(observer))
     observer->observing_count_ -= 1;
 #endif