Always set symbol visibility when compiling with GCC / Clang.

third_party/google_toolbox_for_mac/BUILD.gn

 # Copyright 2014 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.
 
 config("google_toolbox_for_mac_config") {
   include_dirs = [
     ".",
     "src",
     "src/AppKit",
     "src/DebugUtils",
@@ skipping 184 matching lines @@
   }
 
   if (is_component_build) {
     # GTM is third-party code, so we don't want to add _EXPORT
     # annotations to it, so build it without -fvisibility=hidden
     # (else the interface class symbols will be hidden in a 64bit
     # build). Only do this in a component build, so that the shipping
     # chrome binary doesn't end up with unnecessarily exported
     # symbols.
     configs -= [ "//build/config/gcc:symbol_visibility_hidden" ]
+    configs += [ "//build/config/gcc:symbol_visibility_default" ]
   }
 }