Revert "Don't null the RenderView during document destruction"

This reverts http://src.chromium.org/viewvc/blink/?view=revision&revision=161121

This apparently caused a bunch of SVG perf regressions.

TBR=sonnyrao,pdr
BUG=314428

Review URL: https://codereview.chromium.org/58193002

git-svn-id: svn://svn.chromium.org/blink/trunk@161268 bbb929c8-8fbe-4397-9dbb-9b2b20218538