In DateTimeFieldElement::DefaultKeyboardEventHandler, we will get a
crash if GetComputedStyle() is called, but returns null. This could
happen for an element where the style does not need to be displayed.
To fix that, we should use the function EnsureComputedStyle() instead.

(cherry picked from commit b227311de4f90092e0527f5f4624c44717854175)

Change-Id: Icb1700f904fcbc0f4cbea78fbdb824b42f7d8149
Bug: 1489714
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4917073
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1206408}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4923273
Reviewed-by: Daniel Yip <danielyip@google.com>
Owners-Override: Daniel Yip <danielyip@google.com>
Cr-Commit-Position: refs/branch-heads/5993@{#1216}
Cr-Branched-From: 511350718e646be62331ae9d7213d10ec320d514-refs/heads/main@{#1192594}