Remove ScrollView::platformContentsSize

Remove ScrollView::platformContentsSize
https://bugs.webkit.org/show_bug.cgi?id=68188

Reviewed by Darin Adler.

../WebCore:

Since ScrollView keeps track of the contents size in ScrollView::m_contentsSize, we never
have to ask the underlying platform scroll view for contents size since it should always just
be equal to m_contentsSize.

* platform/ScrollView.cpp:
(WebCore::ScrollView::contentsSize):
(WebCore::ScrollView::wheelEvent):
* platform/ScrollView.h:
* platform/mac/ScrollViewMac.mm:
* platform/wx/ScrollViewWx.cpp:

../WebKit/mac:

When creating the document view, make sure it's zero sized which matches the initial size
of the ScrollView. This is fine to do early since DocumentWriter::begin sets the contents
size to a zero size anyway.

* WebView/WebFrameView.mm:
(-[WebFrameView _makeDocumentViewForDataSource:]):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95570 268f45cc-cd09-0410-ab3c-d52691b4dbfc