[WK2] Add Page Visibility API support

[WK2] Add Page Visibility API support
https://bugs.webkit.org/show_bug.cgi?id=81154

Patch by Jesus Sanchez-Palencia <jesus.palencia@openbossa.org> on 2012-04-04
Reviewed by Kenneth Rohde Christiansen.

Use WebPageProxy::viewStateDidChange to correctly define the page's
visibilityState and then set it to the WebProcess. The WebKit2's based
ports just need to implement the related PageClient functions in order
to get this API enabled and working properly.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy):
(WebKit::WebPageProxy::initializeWebPage):
(WebKit::WebPageProxy::viewStateDidChange):
* WebProcess/WebPage/WebPage.cpp:
(WebKit):
(WebKit::WebPage::setVisibilityState):
* WebProcess/WebPage/WebPage.h:
(WebPage):
* WebProcess/WebPage/WebPage.messages.in:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113181 268f45cc-cd09-0410-ab3c-d52691b4dbfc