Disable implicit conversions of scoped_refptr to T* on CrOS.

The implicit conversion has already been disabled for Linux builds for
some time now. https://codereview.chromium.org/510323002 explains the
rationale for proceeding incrementally.

BUG=110610

Review URL: https://codereview.chromium.org/572713003

Cr-Commit-Position: refs/heads/master@{#295234}

base/memory/ref_counted.h

@@ -17,7 +17,7 @@
 #include "base/threading/thread_collision_warner.h"
 #include "build/build_config.h"
 
-#if defined(OS_LINUX) && !defined(OS_CHROMEOS)
+#if defined(OS_LINUX)
 #define DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR
 #endif