Open-panel image transcoding routed to the requesting iframe's process

Component: WebKit UIProcess | d0321d4

In WebKit's Site Isolation model, each cross-site iframe can be hosted in its own WebProcess. The UIProcess mediates file-picker interactions and, on macOS, may transcode chosen images before granting sandbox extensions and delivering file URLs via IPC. Historically this delivery assumed one web process per page (legacyMainFrameProcess()), breaking the process-per-site invariant. This commit makes didChooseFilesForOpenPanelWithImageTranscoding take the requesting WebProcessProxy& explicitly and use it for sandbox extension grants and IPC sends.

Significance

File-input results and their sandbox extensions now target the specific process that owns the requesting frame, fixing cross-site iframes silently failing to receive user-selected files.

Audit directions

Forward-facing: any UIProcess delivery path still using legacyMainFrameProcess() for sandbox-extension grants or file/URL delivery can misroute a capability to the wrong process under Site Isolation. Audit the remaining runOpenPanel/didChooseFilesForOpenPanel siblings and other sandbox-extension grant sites for legacyMainFrameProcess() usage. The tell: a legacyMainFrameProcess() call adjacent to a grantSandboxExtension/file-URL IPC send.

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