Revert 161127 "Don't leave stale pointer into m_grid"

It's causing http://crbug.com/314574.

> Don't leave stale pointer into m_grid
> 
> This change fixes several stale pointers issue in our code:
> - We would wrongly add boxes wrapped in anonymous wrappers to
> m_grid, which would never be removed.
> - RenderBlock::removeLeftoverAnonymousBlock would remove some
> anonymous wrappers under RenderGrid without notifying it of
> the hierarchy change (yay!).
> 
> To fix the second issue, I took the more conservative approach
> of notifying RenderGrid of any child's death and not clearing
> parent in removeLeftoverAnonymousBlock.
> 
> BUG=313293
> 
> Review URL: https://codereview.chromium.org/49383012

TBR=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/53783007

git-svn-id: svn://svn.chromium.org/blink/trunk@161291 bbb929c8-8fbe-4397-9dbb-9b2b20218538