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

build/config/gcc/BUILD.gn

Index: build/config/gcc/BUILD.gn
diff --git a/build/config/gcc/BUILD.gn b/build/config/gcc/BUILD.gn
index b0fc9812cf5153eb7dc7dd636a47b8815db7a47d..43162b608167157ff382de1c03b611c4da6506f0 100644
--- a/build/config/gcc/BUILD.gn
+++ b/build/config/gcc/BUILD.gn
@@ -20,6 +20,18 @@ config("symbol_visibility_hidden") {
   cflags = [ "-fvisibility=hidden" ]
 }
 
+# Set the appropriate default symbol visibility based on existing
+# configurations.
+config("default_symbol_visibility") {
+  # XCTests inject a dynamic library into an iOS application. If fvisibility
+  # is set to hidden, then some symbols from the application needed by XCTests
+  # are not available. Make sure all symbols are exported for Debug
+  # configuration on iOS so XCTests will work.
+  if (!is_ios || is_release) {
+    configs = [ ":symbol_visibility_hidden" ]
+  }
+}
+
 # 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.