base: Change DCHECK_IS_ON to a macro DCHECK_IS_ON() to match Chromium

changes (see http://crrev.com/e649f573) and fix unit test runtime
errors when building with GYP_DEFINES=dcheck_always_on=1.

libcef_dll/base/cef_thread_checker_impl.cc

@@ -5,6 +5,7 @@
 #include "include/base/internal/cef_thread_checker_impl.h"
 
 namespace base {
+namespace cef_internal {
 
 ThreadCheckerImpl::ThreadCheckerImpl()
     : valid_thread_id_() {
@@ -31,4 +32,5 @@ void ThreadCheckerImpl::EnsureThreadIdAssigned() const {
   }
 }
 
+}  // namespace cef_internal
 }  // namespace base