Disable layout optimization when column height may be non-uniform.

We have no way of telling what changes beyond the first column break, so if we
cannot guarantee that the column height *is* and *was* non-uniform, we need to
re-lay out children that may stretch into the unknown.

Review-Url: https://codereview.chromium.org/2562273003
Cr-Commit-Position: refs/heads/master@{#437928}
5 files changed