Fix rounding in paintSelection

Fix rounding in paintSelection
https://bugs.webkit.org/show_bug.cgi?id=86693

Reviewed by Eric Seidel.

Break out rounding logic from InlineTextBox::paintSelection into separate
function and use it for all the EllipsisBox paintSelection implementation
and selection gap calculation. This ensures that selections are painted
without gaps and overlaps once we turn on subpixel layout.

No new tests, covered by existing tests in editing/selection and
editing/style.

* rendering/EllipsisBox.cpp:
(WebCore::EllipsisBox::paintSelection):
* rendering/InlineTextBox.cpp:
(WebCore::alignSelectionRectToDevicePixels):
(WebCore):
(WebCore::InlineTextBox::paintSelection):
* rendering/InlineTextBox.h:
(WebCore):
* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::logicalLeftSelectionGap):
(WebCore::RenderBlock::logicalRightSelectionGap):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117491 268f45cc-cd09-0410-ab3c-d52691b4dbfc