Fix for performance regression on high dpi devices.

ComputedStyle::getRoundedInnerBorderFor() could generate negative
content boxes for boxes with no content and sub pixel borders that
round up (eg. 1.5px-1.99px).  Negative content boxes are illegal per
spec, and causes BoxBorderPainter::paintBorderFastPath() to bail,
and performance to be substantially degraded.

This issue would be evident on the paint-offset-changes perftest when
device pixel ratio was set to eg. 1.5.  The issue was introduced in
934becac

BUG=692955

Review-Url: https://codereview.chromium.org/2782153002
Cr-Commit-Position: refs/heads/master@{#460534}

third_party/WebKit/Source/core/style/ComputedStyle.cpp

@@ -1490,7 +1490,9 @@ FloatRoundedRect ComputedStyle::getRoundedInnerBorderFor(
     bool includeLogicalRightEdge) const {
   LayoutRect innerRect(borderRect);
   innerRect.expand(insets);
-  innerRect.size().clampNegativeToZero();
+  LayoutSize innerRectSize = innerRect.size();
+  innerRectSize.clampNegativeToZero();
+  innerRect.setSize(innerRectSize);
 
   FloatRoundedRect roundedRect(pixelSnappedIntRect(innerRect));