Don't hide V8 symbols in debug builds

That way, backtraces should work

BUG=
R=machenbach@chromium.org

Review-Url: https://codereview.chromium.org/2168593002
Cr-Commit-Position: refs/heads/master@{#37905}

BUILD.gn

@@ -31,9 +31,6 @@ declare_args() {
   # Embeds the given script into the snapshot.
   v8_embed_script = ""
 
-  # Support for backtrace_symbols on linux.
-  v8_enable_backtrace = false
-
   # Sets -dENABLE_DISASSEMBLER.
   v8_enable_disassembler = false
 

gni/v8.gni

@@ -16,6 +16,9 @@ declare_args() {
   # Turns on compiler optimizations in V8 in Debug build.
   v8_optimized_debug = true
 
+  # Support for backtrace_symbols on linux.
+  v8_enable_backtrace = ""
+
   # Enable the snapshot feature, for fast context creation.
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
   v8_use_snapshot = true
@@ -31,6 +34,10 @@ if (v8_use_external_startup_data == "") {
   v8_use_external_startup_data = v8_use_snapshot && !is_ios
 }
 
+if (v8_enable_backtrace == "") {
+  v8_enable_backtrace = is_debug && !v8_optimized_debug
+}
+
 ###############################################################################
 # Templates
 #
@@ -63,6 +70,11 @@ if (is_debug && !v8_optimized_debug) {
   }
 }
 
+if (is_posix && v8_enable_backtrace) {
+  v8_remove_configs += [ "//build/config/gcc:symbol_visibility_hidden" ]
+  v8_add_configs += [ "//build/config/gcc:symbol_visibility_default" ]
+}
+
 # All templates should be kept in sync.
 template("v8_source_set") {
   source_set(target_name) {