Skip invalidations for zero size objects.

When doing an incremental invalidation we can endup issuing the same
invalidation twice if either of the new bounds or old bounds is zero sized.

This CL adds a check so we can just issue one invalidation in case either of
the bounds are zero sized and exit early.

BUG=320139

Review URL: https://codereview.chromium.org/235983004

git-svn-id: svn://svn.chromium.org/blink/trunk@171862 bbb929c8-8fbe-4397-9dbb-9b2b20218538