The optimized code for TestSetWithCustomIterator holds a weak reference to the map
for the entries object. If this is collected by the GC then the optimized code deopts
which causes the test to fail. To prevent this, hold onto an entires object to keep
it's map alive.

Change-Id: I5796e74fc1d7c5061bf8c98f7a82fe582d6be76a
Reviewed-on: https://chromium-review.googlesource.com/c/1357043Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57978}