Avoid dangling ptr in TitledUrlIndexTest.GetResultsSortedByTypedCount

Unbind the index() from the node sorter, thereby destroying it, before
the local clietn that is using goes out of scope.

Bug: 1401495
Change-Id: Ifd4683b9539c52231a9ac7b052e90c473cbd8e6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549774


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147340}