Assert that override sizes are only ever set to non-negative values.

This exposed a bug in RenderBlock::fitBorderToLinesIfNeeded where
we would set the override width to a negative value.
When we adjust the right edge, we need to used the adjusted
left value instead of the unadjusted leftEdge value.

Review URL: https://codereview.chromium.org/130473003

git-svn-id: svn://svn.chromium.org/blink/trunk@166730 bbb929c8-8fbe-4397-9dbb-9b2b20218538