[css-typed-om] computedStyleMap should be cached on Element

According to the spec[1], Element.computedStyleMap()
should return the same object each time.

[1]https://drafts.css-houdini.org/css-typed-om-1/#dom-element-computedstylemap

Bug: 816761
Change-Id: Ice74cca345ba5b4f7136f44695a4b3f6ab605f9a
Reviewed-on: https://chromium-review.googlesource.com/939123
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#540969}
6 files changed