Fix selection residue in sub-pixel positioned element

We are inconsistent when invalidating and painting sub-pixel positioned
selections. See the bug for details.

This change add back the enclosingIntRect() calls that were removed in
http://src.chromium.org/viewvc/blink?view=rev&revision=177408. This is
needed before we fix the inconsistency.

BUG=394698
TEST=editing/selection/subpixel-positioned-selection.html

Review URL: https://codereview.chromium.org/464143003

git-svn-id: svn://svn.chromium.org/blink/trunk@180477 bbb929c8-8fbe-4397-9dbb-9b2b20218538