Don't add empty rects during addFocusRingRects

We don't add empty rects during addFocusRingRects except during
generateLineBoxRects in RenderInline::addFocusRingRects.

Empty rects in focus ring rects causes small circle drawn around
the top-left corner of the empty rects on Mac.

Added AbsoluteRectsIgnoringEmptyRectsGeneratorContext for
addFocusRingRects to ignore empty line box rects.

BUG=405791

Review URL: https://codereview.chromium.org/492433006

git-svn-id: svn://svn.chromium.org/blink/trunk@180864 bbb929c8-8fbe-4397-9dbb-9b2b20218538