Reviewed by Geoff.

    Reviewed by Geoff.

        <rdar://problem/5205358> REGRESSION (r21367): All messages appear entirely blank when running Mail off of tip of tree WebKit

        The fix is to return nil from [WebFrame dataSource] when it has not loaded anything but the fake
        empty initial document. However, WebKit still needs the real data source internally, so I also
        added a [WebFrame _dataSource] method that skips this check, and made WebKit use it throughout.

        * Misc/WebNSAttributedStringExtras.mm:
        (fileWrapperForElement):
        * Plugins/WebBaseNetscapePluginView.mm:
        (-[WebBaseNetscapePluginView dataSource]):
        * Plugins/WebNullPluginView.mm:
        (-[WebNullPluginView viewDidMoveToWindow]):
        * Plugins/WebPluginController.mm:
        (-[WebPluginController URLPolicyCheckReferrer]):
        * WebCoreSupport/WebFrameBridge.mm:
        (-[WebFrameBridge dataSource]):
        (-[WebFrameBridge redirectDataToPlugin:]):
        * WebCoreSupport/WebFrameLoaderClient.mm:
        (WebFrameLoaderClient::makeDocumentView):
        (WebFrameLoaderClient::forceLayoutForNonHTML):
        (WebFrameLoaderClient::prepareForDataSourceReplacement):
        (WebFrameLoaderClient::canCachePage):
        * WebCoreSupport/WebViewFactory.mm:
        (-[WebViewFactory bridgeForView:]):
        * WebKit.xcodeproj/project.pbxproj:
        * WebView/WebArchiver.mm:
        (+[WebArchiver archiveFrame:]):
        (+[WebArchiver archiveMainResourceForFrame:]):
        (+[WebArchiver _archiveWithMarkupString:fromFrame:nodes:]):
        (+[WebArchiver archiveSelectionInFrame:]):
        * WebView/WebFrame.mm:
        (-[WebFrame _loadURL:referrer:intoChild:]):
        (-[WebFrame _addChild:]):
        (-[WebFrame _dataSource]):
        (-[WebFrame DOMDocument]):
        (-[WebFrame dataSource]):
        * WebView/WebFrameInternal.h:
        * WebView/WebHTMLView.mm:
        (-[WebHTMLView _setPrinting:minimumPageWidth:maximumPageWidth:adjustViewSize:]):
        * WebView/WebRenderNode.mm:
        (-[WebRenderNode initWithWebFrameView:]):
        * WebView/WebView.mm:
        (-[WebView _mainFrameOverrideEncoding]):
        (-[WebView mainFrameURL]):
        (-[WebView mainFrameTitle]):
        (-[WebView mainFrameIcon]):
        (-[WebView validateUserInterfaceItemWithoutDelegate:]):
        (-[WebView replaceSelectionWithArchive:]):
        (-[WebView _isLoading]):
        (-[WebView _performTextSizingSelector:withObject:onTrackingDocs:selForNonTrackingDocs:newScaleFactor:]):
        (-[WebView _notifyTextSizeMultiplierChanged]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@21618 268f45cc-cd09-0410-ab3c-d52691b4dbfc