Implement tagging of profiler log event blocks.

src/checks.h

Index: src/checks.h
diff --git a/src/checks.h b/src/checks.h
index 3b0c85135475f3d028694ad5d962d004d88218c0..eeb748b4a80d875e16234a4a20b14575b33a7be9 100644
--- a/src/checks.h
+++ b/src/checks.h
@@ -125,7 +125,9 @@ static inline void CheckEqualsHelper(const char* file,
                                      const char* expected,
                                      const char* value_source,
                                      const char* value) {
-  if (strcmp(expected, value) != 0) {
+  if ((expected == NULL && value != NULL) ||
+      (expected != NULL && value == NULL) ||
+      (expected != NULL && value != NULL && strcmp(expected, value) != 0)) {
     V8_Fatal(file, line,
              "CHECK_EQ(%s, %s) failed\n#   Expected: %s\n#   Found: %s",
              expected_source, value_source, expected, value);