Use new downcast helpers for blink::SpinButtonElement

Bug: 891908
Change-Id: I2107c9975d3605a4937d4e6946d8ea716fab2789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652391
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668491}
2 files changed