Element::focus() should acquire the ownership of Frame.

Element::focus() should acquire the ownership of Frame.
https://bugs.webkit.org/show_bug.cgi?id=150204
<rdar://problem/23136794>

Reviewed by Brent Fulgham.

Source/WebCore:

The FrameSelection::setSelection method sometimes releases the last reference to a frame.
When this happens, the Element::updateFocusAppearance would attempt to use dereferenced memory.
Instead, we should ensure that the Frame lifetime is guaranteed to extend through the duration
of the method call.

Test: editing/selection/focus-iframe-removal-crash.html

* dom/Element.cpp:
(WebCore::Element::updateFocusAppearance):

LayoutTests:

* editing/selection/focus-iframe-removal-crash-expected.txt: Added.
* editing/selection/focus-iframe-removal-crash.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@192433 268f45cc-cd09-0410-ab3c-d52691b4dbfc