Don't set -fvisibility=hidden for iOS Debug.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 456 matching lines @@
     "//build/config/win:winver",
   ]
 }
 if (current_os == "winrt_81" || current_os == "winrt_81_phone" ||
     current_os == "winrt_10") {
   _native_compiler_configs += [ "//build/config/win:target_winrt" ]
 }
 if (is_posix) {
   _native_compiler_configs += [
     "//build/config/gcc:no_exceptions",
-    "//build/config/gcc:symbol_visibility_hidden",
+    "//build/config/gcc:default_symbol_visibility",
   ]
 }
 
 if (is_android) {
   _native_compiler_configs +=
       [ "//build/config/android:default_cygprofile_instrumentation" ]
 }
 
 if (is_clang && !is_nacl) {
   _native_compiler_configs += [
@@ skipping 97 matching lines @@
     forward_variables_from(invoker, "*")
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     if (!defined(deps)) {
       deps = []
     }
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }