Always set symbol visibility when compiling with GCC / Clang.

build/config/gcc/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.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
 
 # This config causes functions not to be automatically exported from shared
 # libraries. By default, all symbols are exported but this means there are
 # lots of exports that slow everything down. In general we explicitly mark
 # which functiosn we want to export from components.
 #
 # Some third_party code assumes all functions are exported so this is separated
 # into its own config so such libraries can remove this config to make symbols
 # public again.
 #
 # See http://gcc.gnu.org/wiki/Visibility
 config("symbol_visibility_hidden") {
   # Note that -fvisibility-inlines-hidden is set globally in the compiler
   # config since that can almost always be applied.
   cflags = [ "-fvisibility=hidden" ]
 }
 
+# This config is usually set when :symbol_visibility_hidden is removed.
+# It's often a good idea to set visibility explicitly, as there're flags
+# which would error out otherwise (e.g. -fsanitize=cfi-unrelated-cast)
+config("symbol_visibility_default") {
+  cflags = [ "-fvisibility=default" ]
+}
+
 # The rpath is the dynamic library search path. Setting this config on a link
 # step will put the directory where the build generates shared libraries into
 # the rpath.
 #
 # It's important that this *not* be used for release builds we push out.
 # Chrome uses some setuid binaries, and hard links preserve setuid bits. An
 # unprivileged user could gain root privileges by hardlinking a setuid
 # executable and then adding in whatever binaries they want to run into the lib
 # directory.
 #
@@ skipping 62 matching lines @@
     if (current_cpu == "mipsel") {
       ldflags += [ "-pie" ]
     }
   }
 }
 
 config("no_exceptions") {
   cflags_cc = [ "-fno-exceptions" ]
   cflags_objcc = cflags_cc
 }