[css-contain] Size containment should include scrollbars

Like it happens on regular elements,
flexbox and grid containers with "contain: size"
should include scrollbars width when computing the intrinsic sizes.

BUG=948300
TEST=external/wpt/css/css-contain/contain-size-scrollbars-001.html
TEST=external/wpt/css/css-contain/contain-size-scrollbars-002.html
TEST=external/wpt/css/css-contain/contain-size-scrollbars-003.html

Change-Id: If044632469e3c2500aac155d04f9a319930ef6e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547797
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Javier Fernandez <jfernandez@igalia.com>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#646532}