Make StyleSharingList store raw ptrs

The list only makes sense in 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.

I tried this before and it was reverted because we call into
styleForElement outside recalcStyle in some cases and if you did that
and then removed the Element from the page it'd leave a stale pointer
in the list.

Review URL: https://codereview.chromium.org/42963002

git-svn-id: svn://svn.chromium.org/blink/trunk@161322 bbb929c8-8fbe-4397-9dbb-9b2b20218538