Use branch prediction hints to make trace events cheaper

base/compiler_specific.h

Index: base/compiler_specific.h
diff --git a/base/compiler_specific.h b/base/compiler_specific.h
index 2ba34da1cc4bbfef27264de7b34ece19628ef1b0..cbb60a0d6783e4d7f72f5aae449b3460eb6a3005 100644
--- a/base/compiler_specific.h
+++ b/base/compiler_specific.h
@@ -218,4 +218,22 @@
 #endif  // defined(OS_WIN)
 #endif  // !defined(CDECL)
 
+// Macro for hinting that an expression is likely to be true.
+#if !defined(LIKELY)
+#if defined(COMPILER_GCC)
+#define LIKELY(x) __builtin_expect(!!(x), 1)
+#else
+#define LIKELY(x) (x)
+#endif  // defined(COMPILER_GCC)
+#endif  // !defined(LIKELY)
+
+// Macro for hinting that an expression is likely to be false.
+#if !defined(UNLIKELY)
+#if defined(COMPILER_GCC)
+#define UNLIKELY(x) __builtin_expect(!!(x), 0)
+#else
+#define UNLIKELY(x) (x)
+#endif  // defined(COMPILER_GCC)
+#endif  // !defined(UNLIKELY)
+
 #endif  // BASE_COMPILER_SPECIFIC_H_