https://bugs.webkit.org/show_bug.cgi?id=49211

Reviewed by Adam Roben.

DownloadProxy will be the implementation of the forthcoming WKDownloadRef object.

* Shared/APIObject.h:
Add TypeDownload.

* UIProcess/Downloads/DownloadProxy.cpp:
(WebKit::generateDownloadID):
Generate a unique download ID.

(WebKit::DownloadProxy::invalidate):
Null out the web context.

* UIProcess/Downloads/DownloadProxy.h:
(WebKit::WebContext::processDidClose):
Invalidate all downloads.

(WebKit::WebContext::createDownloadProxy):
Create a WebDownload object.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::receivedPolicyDecision):
Call createDownloadProxy instead.

* WebKit2.pro:
* WebKit2.xcodeproj/project.pbxproj:
* win/WebKit2.vcproj:
Add new files.

* win/WebKit2Common.vsprops:
Add new include path.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@71574 268f45cc-cd09-0410-ab3c-d52691b4dbfc