Add a "preview" state to Page Visibility API implementation

Add a "preview" state to Page Visibility API implementation
https://bugs.webkit.org/show_bug.cgi?id=81355

Patch by Jesus Sanchez-Palencia <jesus.palencia@openbossa.org> on 2012-03-30
Reviewed by Adam Barth.

Updating the Page Visibility API implementation to the current spec version.

Source/WebCore:

This change is covered by fast/events/page-visibility-transition-test.html,
so no new tests needed.

* page/PageVisibilityState.cpp:
(WebCore::pageVisibilityStateString):
* page/PageVisibilityState.h:

Source/WebKit/chromium:

* public/WebPageVisibilityState.h:
* src/AssertMatchingEnums.cpp:
* src/WebViewImpl.cpp:
(WebKit::WebViewImpl::setVisibilityState):

Source/WebKit/efl:

* WebCoreSupport/AssertMatchingEnums.cpp:
* ewk/ewk_view.h:

Tools:

* DumpRenderTree/chromium/LayoutTestController.cpp:
(LayoutTestController::setPageVisibility):

LayoutTests:

* fast/events/page-visibility-transition-test-expected.txt:
* fast/events/page-visibility-transition-test.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@112664 268f45cc-cd09-0410-ab3c-d52691b4dbfc