Introduce a flag for tracing retaining path in GC.

build/features.gypi

Index: build/features.gypi
diff --git a/build/features.gypi b/build/features.gypi
index a3ee0fbbf08cd52e768984be9403ad8426c7eff3..b3c47fcc7784f7cfb6071324e082f52b9d7a30e3 100644
--- a/build/features.gypi
+++ b/build/features.gypi
@@ -39,6 +39,8 @@
 
     'v8_trace_maps%': 0,
 
+    'v8_trace_retaining_path%': 0,
+
     'v8_use_snapshot%': 'true',
 
     'v8_enable_verify_predictable%': 0,
@@ -79,6 +81,9 @@
       ['v8_trace_maps==1', {
         'defines': ['TRACE_MAPS',],
       }],
+      ['v8_trace_retaining_path==1', {
+        'defines': ['TRACE_RETAINING_PATH',],
+      }],
       ['v8_enable_verify_predictable==1', {
         'defines': ['VERIFY_PREDICTABLE',],
       }],