Fix float-cast-overflow in MenuListIntrinsicInlineSize()

Converts max_option_width to LayoutUnit instead of casting to int.
The LayoutUnit constructor uses base::saturated_cast<> internally.

Bug: 1068338
Change-Id: Id13f98c440eaad1ba2fad8ad0ae84129121c734c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138672
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756960}