Revert "Cleanup deadcode in Rendering"

This reverts commit 25a48eab31ad4f0a54bcaea1ead2b0cc4dec74d2 [1]
with the exception of the changes in RenderFlowThread which have
additional changes on top of them and do not cleanly revert.

This is a speculative rollout, see crbug.com/402059.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=179521

BUG=402059
TBR=leviw
NOTRY=true

Review URL: https://codereview.chromium.org/463633002

git-svn-id: svn://svn.chromium.org/blink/trunk@179989 bbb929c8-8fbe-4397-9dbb-9b2b20218538