[CSS Regions] Selection dragged from a region paints its background

[CSS Regions] Selection dragged from a region paints its background
https://bugs.webkit.org/show_bug.cgi?id=117607

Reviewed by Alexandru Chiculita.

.:

Add a manual test for the painting of a dragged selection from a region. We don't have an
automated way to test this yet. This test is similar to the other manual selection dragging
tests (e.g. ManualTests/drag-image.html).

* ManualTests/regions/drag-selection-painting.html: Added.

Source/WebCore:

When a RenderFlowThread is in the selection paint phase, tell its layer to paint with the
selection only behavior. This way, only foregrounds and not backgrounds will be painted in
the dragged selection.

Manual test: ManualTests/regions/drag-selection-painting.html

* rendering/RenderFlowThread.cpp:
(WebCore::RenderFlowThread::paintFlowThreadPortionInRegion):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@151571 268f45cc-cd09-0410-ab3c-d52691b4dbfc