The TryMigrateInstance should be passed the instance object to migrate,
not the map of the object. Also make the runtime function explicitly
check for JSObjects.

BUG=v8:9684

Change-Id: I03605d9f3103b618243c12ad0b63035484ef4134
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2487270
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70731}