[CSS Regions] Selecting text through nested regions causes weird and unclearable selection  
https://bugs.webkit.org/show_bug.cgi?id=105641

Reviewed by David Hyatt.

If you have a region with a nested region inside what is rendered below the base region and you are trying select text
starting from the base region and ending it in the nested region, firstly you got a weird selection, secondly you can't
clear the selection. We could prevent this strange behavior by preventing the selection through different region flows.

Source/WebCore: 

Test: fast/regions/selecting-text-through-different-region-flows.html

* rendering/RenderView.cpp:
(WebCore::RenderView::setSelection): Don't allow selection when trying to select different region flows.

LayoutTests: 

* fast/regions/selecting-text-through-different-region-flows-expected.txt: Added.
* fast/regions/selecting-text-through-different-region-flows.html: Added.
* platform/chromium-mac/fast/regions/selecting-text-through-different-region-flows-expected.png: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139197 268f45cc-cd09-0410-ab3c-d52691b4dbfc