don't move on rubber-band
https://bugs.webkit.org/show_bug.cgi?id=124745

Reviewed by Tim Horton.

Source/WebCore: 

New setting backgroundShouldExtendBeyondPage() will cause the tile cache to have a 
margin, and it will also cause fixed elements and backgrounds to stick to the 
viewport on scroll instead of sticking to the document. 

* WebCore.exp.in:
* page/FrameView.cpp:
(WebCore::FrameView::scrollBehaviorForFixedElements):
* page/Settings.in:
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::RenderLayerBacking):

Source/WebKit: 

Keep Windows happy.

* WebKit.vcxproj/WebKitExportGenerator/WebKitExports.def.in:

Source/WebKit2: 

Add SPI to enable the new setting.

* Shared/WebPreferencesStore.h:
* UIProcess/API/C/WKPreferences.cpp:
(WKPreferencesSetBackgroundShouldExtendBeyondPage):
(WKPreferencesGetBackgroundShouldExtendBeyondPage):
* UIProcess/API/C/WKPreferencesPrivate.h:
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::updatePreferences):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159960 268f45cc-cd09-0410-ab3c-d52691b4dbfc