Migrate static_casts to WindowTreeHostPlatform::GetHostForWindow

Bug: None
Test: CQ
Change-Id: Ib64e6c4b196b8ba54e01eda5e2b9ddad77a04aff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5385589


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280081}