REGRESSION(r190430): Assertion failure in Text::~Text()

REGRESSION(r190430): Assertion failure in Text::~Text()
https://bugs.webkit.org/show_bug.cgi?id=153577

Reviewed by Antti Koivisto.

Source/WebCore:

The bug was caused by destroyRenderTreeIfNeeded exiting early on all HTMLSlotElement as it lacks a render object.
Fixed it by explicitly avoiding the early return when child is a HTMLSlotElement.

Test: fast/shadow-dom/slot-removal-crash-2.html

* dom/ContainerNode.cpp:
(WebCore::destroyRenderTreeIfNeeded):

LayoutTests:

Added a regression test. The test hits an assertion in debug build without the fix.

* fast/shadow-dom/slot-removal-crash-2-expected.txt: Added.
* fast/shadow-dom/slot-removal-crash-2.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@195727 268f45cc-cd09-0410-ab3c-d52691b4dbfc