Revert 160621 "Make StyleSharingList store raw ptrs"

Causing use-after-free on ClusterFuzz.

> Make StyleSharingList store raw ptrs
> 
> The list only exists as long as the inner block of Document::recalcStyle
> and no DOM mutations can occur inside there so it's safe to store raw
> pointers instead of RefPtrs.
> 
> Review URL: https://codereview.chromium.org/42963002

TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/47173003

git-svn-id: svn://svn.chromium.org/blink/trunk@160665 bbb929c8-8fbe-4397-9dbb-9b2b20218538