Relaxing the stop condition to search for a previous continuation chain.

containingBlockIgnoringAnonymous serves as a stop condition of tree visits
(previousInPreOrder). Before the patch, containingBlockIgnoringAnonymous was not
the root RenderObject of the previous continuation chain, and thus
the previous continuation was not cleared properly.

This patch relaxes the stop condition by using isAnonymous() instead of
isAnonymousBlock() so that the tree search can skip all anonymous wrappers
including RenderTableRow or RenderTableCell.

BUG=389285
TEST=unminimizable test from clusterfuzz
R=inferno@chromium.org

Review URL: https://codereview.chromium.org/358133002

git-svn-id: svn://svn.chromium.org/blink/trunk@177122 bbb929c8-8fbe-4397-9dbb-9b2b20218538