Restore the -fvisibility=hidden flag for tcmalloc when using allocator shim.

base/allocator/BUILD.gn

Index: base/allocator/BUILD.gn
diff --git a/base/allocator/BUILD.gn b/base/allocator/BUILD.gn
index 96ccad239a5455f0dc72e07c4a67fe18a03aa151..d25239ae6f849ea320cb44f72ea7c734c7883cd6 100644
--- a/base/allocator/BUILD.gn
+++ b/base/allocator/BUILD.gn
@@ -236,8 +236,16 @@ if (use_allocator == "tcmalloc") {
         "-Wno-unused-result",
       ]
 
-      configs -= [ "//build/config/gcc:symbol_visibility_hidden" ]
-      configs += [ "//build/config/gcc:symbol_visibility_default" ]
+      # Compiling tcmalloc with -fvisibility=default is only necessary when
+      # not using the allocator shim, which provides the correct visibility
+      # annotations for those symbols which need to be exported (see
+      # //base/allocator/allocator_shim_override_glibc_weak_symbols.h and
+      # //base/allocator/allocator_shim_internals.h for the definition of
+      # SHIM_ALWAYS_EXPORT).
+      if (!use_experimental_allocator_shim) {
+        configs -= [ "//build/config/gcc:symbol_visibility_hidden" ]
+        configs += [ "//build/config/gcc:symbol_visibility_default" ]
+      }
 
       ldflags = [
         # Don't let linker rip this symbol out, otherwise the heap&cpu