[TreesInViz] Fix Dangling raw ptr issue in LayerTreeHostTestCrispUpAfterPinchEnds test.

Fix a teardown bug where a test keeps a cached raw_ptr to
LayerTreeHostImpl past ProxyMain::Stop(). Under TreesInViz, the impl
object is destroyed during Stop(), triggering DanglingRawPtr detection
in the test destructor.

Clear the cached impl pointer in AfterTest() before Stop() runs to avoid
the dangling. No behavior change; only pointer lifetime is shortened.

Change-Id: I63211065914232a55256c94f7823a8c5ad56d4f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6876055
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Victor Miura <vmiura@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1505199}