Clean up handling of strings at the WebKit2 API layer.

Reviewed by Anders Carlsson.

WebKit2: 

- Always use the "copy" rule for functions that return WKStringRefs or WKURLRefs.
- Never return a null WKStringRef or WKURLRef.

* UIProcess/API/C/WKAPICast.h:
(toRef):
(toURLRef):
(toCopiedRef):
(toCopiedURLRef):
* UIProcess/API/C/WKBackForwardListItem.cpp:
(WKBackForwardListItemCopyOriginalURL):
(WKBackForwardListItemCopyURL):
(WKBackForwardListItemCopyTitle):
* UIProcess/API/C/WKBackForwardListItem.h:
* UIProcess/API/C/WKFrame.cpp:
(WKFrameIsMainFrame):
(WKFrameCopyProvisionalURL):
(WKFrameCopyURL):
(WKFrameGetPage):
(WKFrameRetain):
(WKFrameRelease):
* UIProcess/API/C/WKFrame.h:
* UIProcess/API/C/WKNavigationData.cpp:
(WKNavigationDataCopyTitle):
(WKNavigationDataCopyURL):
* UIProcess/API/C/WKNavigationData.h:
* UIProcess/API/C/WKPage.cpp:
(WKPageCopyTitle):
* UIProcess/API/C/WKPage.h:
* WebProcess/InjectedBundle/API/c/WKBundleFrame.cpp:
(WKBundleFrameCopyURL):
(WKBundleFrameCopyName):
(WKBundleFrameCopyCounterValue):
(WKBundleFrameCopyMarkerText):
* WebProcess/InjectedBundle/API/c/WKBundleFrame.h:
* WebProcess/InjectedBundle/API/c/WKBundleNode.cpp:
(WKBundleNodeCopyNodeName):
* WebProcess/InjectedBundle/API/c/WKBundlePage.cpp:
(WKBundlePageCopyRenderTreeExternalRepresentation):

WebKitTools: 

* MiniBrowser/mac/BrowserWindowController.m:
(runJavaScriptAlert):
(runJavaScriptConfirm):
(runJavaScriptPrompt):
(didNavigateWithNavigationData):
(-[BrowserWindowController updateProvisionalURLForFrame:]):
* MiniBrowser/mac/WebBundle/WebBundleMain.m:
(_didClearWindowForFrame):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@64172 268f45cc-cd09-0410-ab3c-d52691b4dbfc