[GTK] Add webkit_print_operation_print() to WebKit2 GTK+ API

[GTK] Add webkit_print_operation_print() to WebKit2 GTK+ API
https://bugs.webkit.org/show_bug.cgi?id=76536

Reviewed by Gustavo Noronha Silva.

* UIProcess/API/gtk/WebKitPrintOperation.cpp:
(webkitPrintOperationPrintPagesForFrame): Helper function to call
WebPageProxy::drawPagesForPrinting using the given
GtkPrintSettings and GtkPageSetup.
(webkitPrintOperationRunDialogForFrame): Use
webkitPrintOperationPrintPagesForFrame().
(webkit_print_operation_print): Print directly using current
GtkPrintSettings and GtkPageSetup without showing the print
dialog.
* UIProcess/API/gtk/WebKitPrintOperation.h:
* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Add new symbols.
* UIProcess/API/gtk/tests/GNUmakefile.am:
* UIProcess/API/gtk/tests/TestPrinting.cpp:
(testPrintOperationPrintLoadChanged):
(testPrintOperationPrintDone):
(testPrintOperationPrintPrinter):
(testPrintOperationPrint):
(beforeAll):
(afterAll):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108068 268f45cc-cd09-0410-ab3c-d52691b4dbfc