Fix double-offset issue of RenderObject::positionFromPaintInvalidationContainer()

As currently we only use positionFromPaintInvalidationContainer to
check if it has changed, neither the issue nor the fix visibly affects
any behavior.

The fix is required by https://codereview.chromium.org/399173005.

Also removed roundedIntPoint because now we pass sub-pixel coordinates
to GraphicsLayer for invalidations
(after https://codereview.chromium.org/353403006/).

BUG=395217

Review URL: https://codereview.chromium.org/408483005

git-svn-id: svn://svn.chromium.org/blink/trunk@178503 bbb929c8-8fbe-4397-9dbb-9b2b20218538