Bug 306075 – [Site Isolation] create proxying version of PageAgent based in UIProcess

• Component: Web Inspector
• Status: RESOLVED FIXED

[Blaze Burg]

Design sketch:

1. Create ProxyingPageAgent in UIProcess, owned by WebPageInspectorController. When site-isolation is enabled, this domain takes over dispatch of the Page domain commands coming from the frontend. ProxyingPageAgent maintains the same PageTarget-based API for forward and backward compatibility with all frontends.

2. Create new PageAgentProxy in WebProcess, which duplicates the "instrumenting" of PageAgent including InspectorInstrumentation hooks and a slot in WebCore::InstrumentingAgents. Events (domContentLoaded, etc) get forwarded to the agent in UIProcess and dispatched out to the frontend. An IPC interface between ProxyingPageAgent and PageAgentProxy will provide a way to collect frame info from each WebProcess as needed. Enablement is bootstrapped via IPC::Messages::WebInspectorBackend:EnablePageInstrumentation.

3. Keep legacy PageAgent implementation around as LegacyPageAgent. We'll need to maintain it in the short-term as a fallback for ProxyingPageAgent, and in the longer-term for WebKitLegacy support. It will continue to live only in top-document's WebProcess, attached to PageInspectorController. Commands not yet implemented for ProxyingPageAgent will be forwarded to the LegacyPageAgent.

[Radar WebKit Bug Importer]

<rdar://problem/168716398>

[Blaze Burg]

Pull request: https://github.com/WebKit/WebKit/pull/61177

[EWS]

Committed 313867@main (6fd4b1f654c6): <https://commits.webkit.org/313867@main>

Reviewed commits have been landed. Closing PR #61177 and removing active labels.