Don't paint below image's baseline unless it is selected.

We only need to paint below an inline image's baseline if it is selected,
otherwise we can just paint the image itself.

When an image's selection state changes we need to update the cached
paint rect if it has a layer, i.e. is positioned, so that the correct
area gets painted next time around. This was the 'real' fix for
http://webkit.org/b/22472 - note that the reduction there only ever failed because the image was relatively positioned.

BUG=238449

Review URL: https://codereview.chromium.org/251783004

git-svn-id: svn://svn.chromium.org/blink/trunk@174301 bbb929c8-8fbe-4397-9dbb-9b2b20218538