Reviewed by Anders Carlsson.

* WebCore.exp.in:

WebKit2: Add support for transparent WebKit2 WKViews.

https://webkit.org/b/50785

Reviewed by Anders Carlsson.

* Shared/WebPageCreationParameters.cpp:
(WebKit::WebPageCreationParameters::encode): Encode drawsBackground and drawsTransparentBackground.
(WebKit::WebPageCreationParameters::decode): Decode drawsBackground and drawsTransparentBackground.
* Shared/WebPageCreationParameters.h:
* UIProcess/API/mac/WKView.h:
* UIProcess/API/mac/WKView.mm:
(-[WKView setDrawsBackground:]): Added.
(-[WKView drawsBackground]): Added.
(-[WKView setDrawsTransparentBackground:]): Added.
(-[WKView drawsTransparentBackground]): Added.
(-[WKView drawRect:]): Only fill the background if the WebProcess isn't valid and drawsBackground.
Use white or clear based on setDrawsTransparentBackground.
(-[WKView isOpaque]): Return drawsBackground.
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::WebPageProxy): Initialize m_drawsBackground and m_drawsTransparentBackground.
(WebKit::WebPageProxy::setDrawsBackground): Added. Send a SetDrawsBackground message.
(WebKit::WebPageProxy::setDrawsTransparentBackground): Added. Send a SetDrawsTransparentBackground message.
(WebKit::WebPageProxy::creationParameters): Set drawsBackground and drawsTransparentBackground.
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::drawsBackground): Added. Return m_drawsBackground.
(WebKit::WebPageProxy::drawsTransparentBackground): Added. Return m_drawsTransparentBackground.
* UIProcess/mac/ChunkedUpdateDrawingAreaProxyMac.mm:
(WebKit::ChunkedUpdateDrawingAreaProxy::platformPaint): Use the copy blend mode when drawing a background.
(WebKit::ChunkedUpdateDrawingAreaProxy::drawUpdateChunkIntoBackingStore): Use the copy blend mode to replace existing content.
* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::transitionToCommittedForNewPage): Setup the new view's background color and transparent flag.
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::WebPage): Initialize m_drawsBackground and m_drawsTransparentBackground.
(WebKit::WebPage::setDrawsBackground): Added. Propagate the flag to the FrameViews.
(WebKit::WebPage::setDrawsTransparentBackground): Added. Propagate the background color to the FrameViews.
* WebProcess/WebPage/WebPage.h:
(WebKit::WebPage::drawsBackground): Added. Return m_drawsBackground.
(WebKit::WebPage::drawsTransparentBackground): Added. Return m_drawsTransparentBackground.
* WebProcess/WebPage/WebPage.messages.in: Added SetDrawsBackground and SetDrawsTransparentBackground.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73666 268f45cc-cd09-0410-ab3c-d52691b4dbfc