Checked number is not automatically truncating to float64.

Bug: chromium:761892
Change-Id: I34bd5d7867cd38b2be18cd39a810605603f515e2
Reviewed-on: https://chromium-review.googlesource.com/649513
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47824}