[Squad] Unnecessary use of mutable style in vp propagation.

Also a couple of other places where ComputedStyle could be made
immutable.

Bug: 813068
Change-Id: I7cebac7951adbf65dbb057ef7b75d4ddab62e90c
Reviewed-on: https://chromium-review.googlesource.com/924145
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537340}

• third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp[diff]
• third_party/WebKit/Source/core/dom/Document.cpp[diff]
• third_party/WebKit/Source/core/layout/LayoutBlock.cpp[diff]