[GTK] Add webkit_web_page_get_id() to WebKit2 GTK+ API
https://bugs.webkit.org/show_bug.cgi?id=111938

Reviewed by Anders Carlsson.

Add API to the web extensions API to get the identifier of a web
page.

* UIProcess/API/gtk/docs/webkit2gtk-sections.txt: Add new symbol.
* UIProcess/API/gtk/tests/WebExtensionTest.cpp:
(methodCallCallback): Check the page ID matches the one returned
by webkit_web_page_get_id().
* WebProcess/InjectedBundle/API/gtk/WebKitWebPage.cpp:
(webkit_web_page_get_id):
* WebProcess/InjectedBundle/API/gtk/WebKitWebPage.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148592 268f45cc-cd09-0410-ab3c-d52691b4dbfc