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

i will take this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/787a6768063a046999da99b37b36517f5ea4f51f

commit 787a6768063a046999da99b37b36517f5ea4f51f
Author: Hwanseung Lee <hs1217.lee@samsung.com>
Date: Mon Mar 05 23:24:04 2018

[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}
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/css/cssom/ElementComputedStyleMap.h
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/dom/Document.h
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/dom/Element.cpp
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/dom/Element.h
[modify] https://crrev.com/787a6768063a046999da99b37b36517f5ea4f51f/third_party/WebKit/Source/core/dom/Node.cpp