Add a comment that CHECK should not be used until the performance issue is fixed.

third_party/WebKit/Source/wtf/Assertions.h

Index: third_party/WebKit/Source/wtf/Assertions.h
diff --git a/third_party/WebKit/Source/wtf/Assertions.h b/third_party/WebKit/Source/wtf/Assertions.h
index 6d10dc1b87e7fcc244da2eb308d7711cec9514c0..637d0950412950c106e95dd2b3a5c7e3f33f5d06 100644
--- a/third_party/WebKit/Source/wtf/Assertions.h
+++ b/third_party/WebKit/Source/wtf/Assertions.h
@@ -211,7 +211,10 @@ class WTF_EXPORT ScopedLogger {
 // vulnerability from which execution must not continue even in a release build.
 // Please sure to file bugs for these failures using the security template:
 //    http://code.google.com/p/chromium/issues/entry?template=Security%20Bug
-// RELEASE_ASSERT is deprecated.  We should use CHECK() instead.
+//
+// WARNING: CHECK is slower than RELEASE_ASSERT on MSVC. We should not replace
+// RELEASE_ASSERT with CHECK until the performance issue is fixed
+// (crbug.com/596760).
 #if ENABLE(ASSERT)
 #define RELEASE_ASSERT(assertion) ASSERT(assertion)
 #elif defined(ADDRESS_SANITIZER)