FrameSelection::appearanceUpdateTimerFired should be robust against layout passes...

FrameSelection::appearanceUpdateTimerFired should be robust against layout passes underneath it
https://bugs.webkit.org/show_bug.cgi?id=183395
<rdar://problem/38055732>

Reviewed by Zalan Bujtas.

Source/WebCore:

In the case where a FrameSelection updates its appearance when m_appearanceUpdateTimer is fired, the
FrameSelection's Frame is unprotected, and can be removed by arbitrary script. This patch applies a simple
mitigation by wrapping the Frame in a Ref when firing the appearance update timer.

Test: editing/selection/iframe-update-selection-appearance.html

* editing/FrameSelection.cpp:
(WebCore::FrameSelection::appearanceUpdateTimerFired):

LayoutTests:

Add a new layout test that passes if we didn't crash.

* editing/selection/iframe-update-selection-appearance-expected.txt: Added.
* editing/selection/iframe-update-selection-appearance.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@230513 268f45cc-cd09-0410-ab3c-d52691b4dbfc