Inform the UIProcess when main frame scrollbars change

Inform the UIProcess when main frame scrollbars change
<rdar://problem/8967496>
https://bugs.webkit.org/show_bug.cgi?id=53941

Reviewed by Anders Carlsson.

* UIProcess/API/C/WKPage.cpp:
(WKPageHasHorizontalScrollbar):
(WKPageHasVerticalScrollbar):
* UIProcess/API/C/WKPage.h:
Add API to query whether the main frame has scrollbars.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy):
(WebKit::WebPageProxy::close):
(WebKit::WebPageProxy::processDidCrash):
(WebKit::WebPageProxy::didChangeScrollbarsForMainFrame):
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::hasHorizontalScrollbar):
(WebKit::WebPageProxy::hasVerticalScrollbar):
Store/update main frame scrollbar state.

* UIProcess/WebPageProxy.messages.in:
* WebProcess/WebCoreSupport/WebChromeClient.cpp:
(WebKit::WebChromeClient::contentsSizeChanged):
* WebProcess/WebCoreSupport/WebChromeClient.h:
(WebKit::WebChromeClient::WebChromeClient):
Post message when main frame scrollbar state changes.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@77846 268f45cc-cd09-0410-ab3c-d52691b4dbfc