DocumentLoader::loadMainResource() does not use main document's cache partition name for subframe document contrary to comment

[Daniel Bates]

In DocumentLoader::loadMainResource() there is a comment that we should use the cache partition of the main document when loading a subframe. But we do not do that. Should we do it? Is the comment incorrect?

[[
void DocumentLoader::loadMainResource(ResourceRequest&& request)
{
    static NeverDestroyed<ResourceLoaderOptions> mainResourceLoadOptions(SendCallbacks, SniffContent, BufferData, StoredCredentialsPolicy::Use, ClientCredentialPolicy::MayAskClientForCredentials, FetchOptions::Credentials::Include, SkipSecurityCheck, FetchOptions::Mode::Navigate, IncludeCertificateInfo, ContentSecurityPolicyImposition::SkipPolicyCheck, DefersLoadingPolicy::AllowDefersLoading, CachingPolicy::AllowCaching);
    CachedResourceRequest mainResourceRequest(ResourceRequest(request), mainResourceLoadOptions);
    if (!m_frame->isMainFrame() && m_frame->document()) {
        // If we are loading the main resource of a subframe, use the cache partition of the main document.
        mainResourceRequest.setDomainForCachePartition(*m_frame->document());
...
]]
<https://trac.webkit.org/browser/trunk/Source/WebCore/loader/DocumentLoader.cpp?rev=+230944#L1732>

[Radar WebKit Bug Importer]

<rdar://problem/39740615>