Logger: extract FLAG_log_code related logging into CodeEventLogger class.

src/log.h

Index: src/log.h
diff --git a/src/log.h b/src/log.h
index 285d8734ba0e1d602f86f086d46df26090ab5977..0b0c491dd5b23925e330e495b839d874aa2afe44 100644
--- a/src/log.h
+++ b/src/log.h
@@ -70,6 +70,7 @@ namespace internal {
 // tick profiler requires code events, so --prof implies --log-code.
 
 // Forward declarations.
+class CodeEventLogger;
 class CompilationInfo;
 class CpuProfiler;
 class Isolate;
@@ -452,6 +453,7 @@ class Logger {
   Log* log_;
   LowLevelLogger* ll_logger_;
   JitLogger* jit_logger_;
+  CodeEventLogger* code_logger_;
 
   NameBuffer* name_buffer_;