Null dereference loading Blink layout test editing/execCommand/delete-hidden-crash...

Null dereference loading Blink layout test editing/execCommand/delete-hidden-crash.html
https://bugs.webkit.org/show_bug.cgi?id=149289
<rdar://problem/22746352>

Reviewed by Enrica Casucci.

Source/WebCore:

This is a merge of Blink r176497:
https://codereview.chromium.org/340713003

It ensures the start & end positions in DeleteSelectionCommand::initializePositionData
are editable.

Test: editing/execCommand/delete-hidden-crash.html

* editing/DeleteSelectionCommand.cpp:
(WebCore::DeleteSelectionCommand::initializePositionData):
* editing/Editor.cpp:
(WebCore::Editor::advanceToNextMisspelling):
* editing/htmlediting.cpp:
(WebCore::firstEditablePositionAfterPositionInRoot):
(WebCore::lastEditablePositionBeforePositionInRoot):
These two functions don't make any sense to return VisiblePosition. Change them
to return Position instead. Since there is a viable conversion from Position to
VisiblePosition. It should not change the behavior of any other components depending
on it.
* editing/htmlediting.h:

LayoutTests:

* editing/execCommand/delete-hidden-crash-expected.txt: Added.
* editing/execCommand/delete-hidden-crash.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@192043 268f45cc-cd09-0410-ab3c-d52691b4dbfc