[Site Isolation] Content in cross-origin subframes is missing when copying and pasting

https://bugs.webkit.org/show_bug.cgi?id=319423
rdar://176590191

Reviewed by Abrar Rahman Protyasha.

When copying content that includes cross-origin subframes, content in those subframes is missing
upon pasting into native apps like TextEdit or Notes, which attempt to read `NSAttributedString`s
from the pasteboard.

While there's an existing solution added in https://commits.webkit.org/300189@main to ensure that
web archives capture cross-origin content under site isolation, there's currently no similar
architecture in place for attributed strings, which means we currently just skip over content in
cross-origin subframes.

To fix this, we apply the same approach taken in 300189@main to attributed string conversion; see
comments below for more details.

Test: SiteIsolation.ReadAttributedStringFromPasteboardAfterCopyWithCrossSiteIframe

* Source/WebCore/editing/cocoa/EditorCocoa.mm:
(WebCore::selectionAsAttributedString):

Add a `remoteFrameContent` argument here, that represents `AttributedString`s collected from remote
frames.

(WebCore::populateRichTextDataIfNeeded):

When serializing attributed strings, collect all remote frame IDs that represent frames where we
need to recursively convert into attributed strings.

(WebCore::Editor::writeSelectionToPasteboard):
* Source/WebCore/editing/cocoa/NodeHTMLConverter.h:
(WebCore::attributedString):
* Source/WebCore/editing/cocoa/NodeHTMLConverter.mm:
(HTMLConverter::HTMLConverter):
(HTMLConverter::_processElement):

Slot in attributed strings collected from remote subframes.

(WebCore::attributedString):
* Source/WebCore/page/EditorClient.h:
* Source/WebKit/UIProcess/Cocoa/WebPageProxyCocoa.mm:
(WebKit::WebPageProxy::getAttributedStringsForRemoteFrames):
* Source/WebKit/UIProcess/WebPageProxy.h:
* Source/WebKit/UIProcess/WebPageProxy.messages.in:
* Source/WebKit/WebProcess/WebCoreSupport/WebEditorClient.cpp:
(WebKit::WebEditorClient::collectAttributedStringsForRemoteFrames):

Add a new client hook that takes a list of remote frame IDs, and calls into the UI process to fan
out to all remote frames and convert their content into attributed strings.

* Source/WebKit/WebProcess/WebCoreSupport/WebEditorClient.h:
* Source/WebKit/WebProcess/WebPage/Cocoa/WebPageCocoa.mm:
(WebKit::WebPage::getAttributedStringsForRemoteFrames):
(WebKit::WebPage::getContentsAsAttributedStringForFrames):
* Source/WebKit/WebProcess/WebPage/WebPage.h:
* Source/WebKit/WebProcess/WebPage/WebPage.messages.in:
* Tools/TestWebKitAPI/Tests/WebKit/WKWebView/SiteIsolation.mm:
(TestWebKitAPI::(SiteIsolation, ReadAttributedStringFromPasteboardAfterCopyWithCrossSiteIframe)):

Add an API test to exercise the fix.

Canonical link: https://commits.webkit.org/317283@main