Reviewed by Dan Bernstein.

        Pass the web page proxy to BackingStore::create
        https://bugs.webkit.org/show_bug.cgi?id=52673

        * UIProcess/BackingStore.cpp:
        (WebKit::BackingStore::create):
        Add WebPageProxy parameter.
    
        (WebKit::BackingStore::BackingStore):
        Add WebPageProxy parameter.

        * UIProcess/BackingStore.h:
        Add WebPageProxy member variable.

        * UIProcess/DrawingAreaProxyImpl.cpp:
        (WebKit::DrawingAreaProxyImpl::incorporateUpdate):
        Pass the web page proxy when creating the backing store.

        * UIProcess/mac/BackingStoreMac.mm:
        (WebKit::BackingStore::backingStoreContext):
        Set the default blend mode.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76077 268f45cc-cd09-0410-ab3c-d52691b4dbfc