[GTK][WK2] Primary clipboard should be updated with the current selection in X11...

[GTK][WK2] Primary clipboard should be updated with the current selection in X11 platforms
https://bugs.webkit.org/show_bug.cgi?id=77097

Reviewed by Martin Robinson.

* WebProcess/WebCoreSupport/WebEditorClient.cpp:
(WebKit::WebEditorClient::respondToChangedSelection): Call
setSelectionPrimaryClipboardIfNeeded() to update primary clipboard
in X11 platforms.
* WebProcess/WebCoreSupport/WebEditorClient.h:
* WebProcess/WebCoreSupport/gtk/WebEditorClientGtk.cpp:
(WebKit::collapseSelection): Callback called when clearing
clipboard contents.
(WebKit::WebEditorClient::setSelectionPrimaryClipboardIfNeeded):
Updaye primary clipboard with the current selection.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106000 268f45cc-cd09-0410-ab3c-d52691b4dbfc