Adopt smart pointers in WebProcessProxy.
Pull request: https://github.com/WebKit/WebKit/pull/19674
Committed 269895@main (6c1701ab4621): <https://commits.webkit.org/269895@main> Reviewed commits have been landed. Closing PR #19674 and removing active labels.
<rdar://problem/117635721>
Regressed: Bug 263845 – REGRESSION(269895@main): ASSERTION FAILED: !m_deletionHasBegun for WebProcessPool