Bug 314130 – [Site Isolation] Introduce frameProcessForBackForwardNavigation helper

• Status: RESOLVED
• Resolution: FIXED
• Component: New Bugs

[Basuke Suzuki]

WebPageProxy::processForTheFrameItem() and WebPageProxy::copyFrameStateForBackForwardNavigation() duplicate the same Site Isolation gate logic to decide whether to use the live frame's process or fall back to the legacy main frame process.

Consolidate this into a shared helper WebPageProxy::frameProcessForBackForwardNavigation(frameItem). The helper:
- Returns nullptr when Site Isolation is disabled (the two callers fall back to their default).
- Returns nullptr when the target item has a SuspendedPageProxy (that path is handled by the BFCache restoration logic — avoid picking a stale process here).
- Returns nullptr with a RELEASE_LOG_ERROR if the frame's process is no longer reachable via a RemotePageProxy in the BrowsingContextGroup — a defensive safety-net.
- Otherwise returns the live frame's process.

Both existing call sites are simplified to use the helper.

Prep patch for rdar://173743733 (multi-process BFCache restoration under Site Isolation).

[Basuke Suzuki]

<rdar://problem/176305227>

[Basuke Suzuki]

Pull request: https://github.com/WebKit/WebKit/pull/64314

[EWS]

Committed 312724@main (e3e8e6850c8d): <https://commits.webkit.org/312724@main>

Reviewed commits have been landed. Closing PR #64314 and removing active labels.