dns: Create DnsTask and DnsTaskResultsManager together

Before this CL, we created DnsTaskResultsManager in the constructor
of HostResolverManager::Job. However, there could be a situation
where we attempt insecure DnsTask first, cancels the insecure DnsTask
and resets the DnsTaskResultsManager, then falls back to a secure
DnsTask. In that case, we should create a newDnsTaskResultsManager for
the secure DnsTask. Create DnsTaskResultsManager in StartDnsTask()
to do that.

This is a preparation for fixing the linked bug.

Bug: 403373872
Change-Id: I739528bf2357cf329d67fa29b390d0207bb86090
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6472335
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1449330}