Use "\uFEFF" instead of "." in WebInspector.ViewportControl's gap elements.

When the gap elements have height:0px, the "." text contributes to the
container's height, which produces strange effects during scrolling.

BUG=579444

Review URL: https://codereview.chromium.org/1613703004

Cr-Commit-Position: refs/heads/master@{#370878}
1 file changed