[gn] Set reasonable defaults for debugging

BUG=chromium:474921

Review-Url: https://codereview.chromium.org/2182933002
Cr-Commit-Position: refs/heads/master@{#38048}

BUILD.gn

@@ -32,7 +32,7 @@ declare_args() {
   v8_embed_script = ""
 
   # Sets -dENABLE_DISASSEMBLER.
-  v8_enable_disassembler = false
+  v8_enable_disassembler = ""
 
   # Sets -dENABLE_GDB_JIT_INTERFACE.
   v8_enable_gdbjit = ""
@@ -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 +83,14 @@ if (v8_enable_gdbjit == "") {
   }
 }
 
+# Derived defaults.
+if (v8_object_print == "") {
+  v8_object_print = is_debug && !v8_optimized_debug
+}
+if (v8_enable_disassembler == "") {
+  v8_enable_disassembler = is_debug && !v8_optimized_debug
+}
+
 v8_generated_peephole_source = "$target_gen_dir/bytecode-peephole-table.cc"
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"