InlineFlowBox doesn't expect m_overflow if knownToHaveNoOverflow is
set. Unfortunately LayoutListItem was bypassing the normal overflow
computation and setting m_overflow without knowledge of InlineFlowBox,
yielding to an ASSERT triggering.

This change refactors the API to prevent manipulations of m_overflow
without the knowledge of InlineFlowBox, fixing the ASSERT. However it
is not the right long term fix. LayoutListItem should be fixed to not
do that as it seems very wrong (https://crbug.com/554160).

BUG=546792

Review URL: https://codereview.chromium.org/1415493008

Cr-Commit-Position: refs/heads/master@{#359601}