css: Expose length attribute of CSSKeyframesRule

For CSSKeyframesRule, getter and length was updated in the spec [1].
In Blink, getter had been already defined. According to the WebIDL
spec [2], interfaces that support indexed properties must define an
integer-typed attribute named "length".

This changes getter and exposes length attribute following the
spec [3]

[1] https://github.com/w3c/csswg-drafts/pull/7887
[2] https://webidl.spec.whatwg.org/#idl-indexed-properties
[3] https://drafts.csswg.org/css-animations/#interface-csskeyframesrule-id

Bug: 1502758
Change-Id: I12bfebe39bf4fc87e04ddc5257c75b5ee7ed15e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5069356
Commit-Queue: Euisang Lim <eui-sang.lim@samsung.com>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1262337}