Remove unnecessary release assertion from mixed content checker.

https://bugs.webkit.org/show_bug.cgi?id=258303
<rdar://110766912>

Reviewed by Brent Fulgham.

We now check the entire frame tree for mixed content checks for all
resources loads. An assertion that a document has a frame is no
longer valid in general. This assertion was originally added in
215749@main in an attempt to cover an untestable case. This replaces
the assert with a null check.

* Source/WebCore/loader/MixedContentChecker.cpp:
(WebCore::foundMixedContentInFrameTree):

Originally-landed-as: 259548.841@safari-7615-branch (74f32c2). rdar://113285455
Canonical link: https://commits.webkit.org/266682@main

Source/WebCore/loader/MixedContentChecker.cpp

@@ -55,13 +55,11 @@ static bool foundMixedContentInFrameTree(const LocalFrame& frame, const URL& url
     auto* document = frame.document();
 
     while (document) {
-        RELEASE_ASSERT_WITH_MESSAGE(document->frame(), "An unparented document tried to load or run content with url: %s", url.string().utf8().data());
-
         if (isMixedContent(*document, url))
             return true;
 
         auto* frame = document->frame();
-        if (frame->isMainFrame())
+        if (!frame || frame->isMainFrame())
             break;
 
         auto* abstractParentFrame = frame->tree().parent();