Initialize m_forceSlowPaintPathWithOverflowingCell.

This variable can be accessed in dirtiedRows without being initialized.
Set the variable in the constructor to handle this case.

BUG=411167

Review URL: https://codereview.chromium.org/612923003

git-svn-id: svn://svn.chromium.org/blink/trunk@182941 bbb929c8-8fbe-4397-9dbb-9b2b20218538