[gn] Set reasonable defaults for debugging

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index b737883542b372d4cf7e2b7aa95f6fc45b1483d3..170ec8157fbb64d12246eb49908f718d95493886 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -52,7 +52,7 @@ declare_args() {
   v8_interpreted_regexp = false
 
   # Sets -dOBJECT_PRINT.
-  v8_object_print = false
+  v8_object_print = ""
 
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
@@ -83,6 +83,11 @@ if (v8_enable_gdbjit == "") {
   }
 }
 
+# Derived defaults.
+if (v8_object_print == "") {
+  v8_object_print = is_debug && !v8_optimized_debug

[Yang, 2016/07/26 09:01:20]

also set v8_enable_disassembler please

+}
+
 v8_generated_peephole_source = "$target_gen_dir/bytecode-peephole-table.cc"
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"