[WK2][GTK] WebProcess SIGSEVs due to incorrect clipboard handling

[WK2][GTK] WebProcess SIGSEVs due to incorrect clipboard handling
https://bugs.webkit.org/show_bug.cgi?id=79252

Do not execute clipboard callbacks after the Frame associated with
it is destroyed.

This change is already covered by the TestWebViewEditor unit tests
(among others), they hang (because WebProcess dies) without this
patch in Debug builds.

* WebProcess/WebCoreSupport/gtk/WebEditorClientGtk.cpp:
(EditorClientFrameDestructionObserver):
(WebKit::EditorClientFrameDestructionObserver::EditorClientFrameDestructionObserver):
(WebKit::EditorClientFrameDestructionObserver::frameDestroyed):
(WebKit::EditorClientFrameDestructionObserver::destroyOnClosureFinalization):
(WebKit):
(WebKit::WebEditorClient::setSelectionPrimaryClipboardIfNeeded):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108631 268f45cc-cd09-0410-ab3c-d52691b4dbfc