https://bugs.webkit.org/show_bug.cgi?id=113270

Reviewed by Enrica Casucci.

Source/WebCore:

The bug was caused by updateSelectionForMouseDownDispatchingSelectStart always replacing selection whenever
the target node was inside an element with -webkit-suer-select even when we were attemping to extend selection
in handleMousePressEventSingleClick.

Fixed the bug by extracting this logic as a separate function (expandSelectionToRespectUserSelectAll) and deploying
it in handleMousePressEventSingleClick to extend selection as needed.

Test: editing/selection/user-select-all-with-shift.html

* page/EventHandler.cpp:
(WebCore::expandSelectionToRespectUserSelectAll): Extracted from updateSelectionForMouseDownDispatchingSelectStart.
(WebCore::EventHandler::updateSelectionForMouseDownDispatchingSelectStart):
(WebCore::EventHandler::selectClosestWordFromHitTestResult):
(WebCore::EventHandler::selectClosestWordOrLinkFromMouseEvent):
...