Don't hide V8 symbols in debug builds

gni/v8.gni

Index: gni/v8.gni
diff --git a/gni/v8.gni b/gni/v8.gni
index 9bbc33387681717e4c3370f0953f99006c9fb1e3..5aac4e333a26feae5cb10c2551f8f24b3a260f6d 100644
--- a/gni/v8.gni
+++ b/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

[Michael Achenbach, 2016/07/20 11:55:30]

Consider turning it on by default for all debug builds if the increase in binary
size is not too much.

+}
+
 ###############################################################################
 # 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) {