Only check object size if the bounds don't match.

In r171862 we added a check to do a bounds invalidation if one of the provided
objects has zero size.

We can further improve this by only doing the invalidation if the bounds are
not equal. If they are equal we'll early out of any invalidation with a later
bounds check. Having the bounds check allows us to skip calling
repaintUsingContainer for the zero sized objects. This has no visual effect
as the zero sized invalidations would be skipped at the RenderView layer.

Review URL: https://codereview.chromium.org/246883005

git-svn-id: svn://svn.chromium.org/blink/trunk@172395 bbb929c8-8fbe-4397-9dbb-9b2b20218538