[DIPS] Enable tests to wait for DIPS redirect handling to complete.

1. Make DIPSRedirectContext report empty chains (when no redirect takes
   place)
2. Add DIPSService::Observer, currently only containing a OnChainHandled
   method.
3. Add RedirectChainObserver, a helper class for tests to wait for a
   chain to be handled.
4. Write lots of tests.

Bug: 1412163
Change-Id: I1c2d132a69bff2c5eeeec1781b82298c1fd6c073
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4215052
Reviewed-by: Joshua Hood <jdh@chromium.org>
Commit-Queue: Ryan Tarpine <rtarpine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1100639}