Add another assert to ensure we don't get unexpected negative dimensions

BUG=485931,490423,397449

Change-Id: I169ae0cadb64a898364039948ad87447a6f8990b
Reviewed-on: https://chromium-review.googlesource.com/580130
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488436}

third_party/WebKit/Source/core/layout/LayoutBlock.cpp

@@ -1430,6 +1430,7 @@ void LayoutBlock::ComputePreferredLogicalWidths() {
   }
 
   LayoutUnit border_and_padding = BorderAndPaddingLogicalWidth();
+  DCHECK_GE(border_and_padding, LayoutUnit());
   min_preferred_logical_width_ += border_and_padding;
   max_preferred_logical_width_ += border_and_padding;