Bug 315105 – [Site Isolation] Add WebFrameProxy children manipulation and WebBackForwardCacheEntry caching foundations

• Component: WebKit2
• Status: RESOLVED FIXED

[Basuke Suzuki]

Add the UIProcess-side foundations needed by the upcoming UI-driven iframe
orchestration for same-site BFCache with cross-site iframes:

  - WebFrameProxy::takeChildFrames / adoptChildFrames: API to detach the
    direct iframe children of a frame and reattach them later. Used to move
    the cross-site iframe WebFrameProxys off m_childFrames while the page
    is in BFCache and put them back on restore.

  - WebBackForwardCacheEntry: setCachedChildren / takeCachedChildren /
    hasCachedChildren / referencesIframeProcess. Lets the cache entry hold
    the detached children for the duration of the cache and answer
    process-death questions over the cached subtree.

  - WebBackForwardCache::removeEntriesForProcess: extend the
    process-termination cascade to walk
    WebBackForwardCacheEntry::referencesIframeProcess so an entry whose
    cached subtree references a now-dead WebProcess gets dropped.

  - WebProcess IPCs: rename ClearCachedPage → DiscardCachedPage; add
    DiscardCachedIframe (UI→iframe WP). Both are dispatched from
    ~WebBackForwardCacheEntry to evict the corresponding per-process cached
    state when the entry is destroyed.

This PR adds the API surface and wiring; the call sites that exercise it
(the cache-side walker, restore-side adopt, etc.) land in the feature-enable
PR.

No behavior change for users (cross-site iframe BFCache remains gated and
currently has no orchestration that uses these APIs).

rdar://177439863

[Basuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/65378

[EWS]

Committed 313713@main (e16931a76e07): <https://commits.webkit.org/313713@main>

Reviewed commits have been landed. Closing PR #65378 and removing active labels.