[d8] Fixed external gc test (limit multiplied by number of isolates).

Bug: v8:8108
Change-Id: Iaa388e53f99c827c4bcf5177f1a9e26097f07f11
Reviewed-on: https://chromium-review.googlesource.com/1195362Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Rodrigo Bruno <rfbpb@google.com>
Cr-Commit-Position: refs/heads/master@{#55480}

src/d8.cc

@@ -3400,7 +3400,7 @@ int Shell::Main(int argc, char* argv[]) {
   ShellArrayBufferAllocator shell_array_buffer_allocator;
   MockArrayBufferAllocator mock_arraybuffer_allocator;
   MockArrayBufferAllocatiorWithLimit mock_arraybuffer_allocator_limit(
-      options.mock_arraybuffer_allocator_limit);
+      options.mock_arraybuffer_allocator_limit * options.num_isolates);
   if (options.mock_arraybuffer_allocator_limit > 0) {
     Shell::array_buffer_allocator = &mock_arraybuffer_allocator_limit;
   } else if (options.mock_arraybuffer_allocator) {