If the current context is overwritten by doing Realm.navigate(0) we
fail to delete the module embedder data from the correct current
context, because we have an handle to the old context which was
already cleaned up by calling DisposeRealm in RealmNavigate.

This patch disallows navigation to the first realm.

Bug: chromium:711165
Change-Id: I6b9d3187367dae9d1fe38c0efa361d461c94c917
Reviewed-on: https://chromium-review.googlesource.com/476970Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44656}