FileSelectHelper double-release POC

Platform: macOS only.

Prerequisites: Enterprise Connectors content analysis (FILE_ATTACHED) must be enabled via managed policy so that ContentAnalysisDelegate::IsEnabled() returns true and an async deep scan is started for the uploaded file.

1. Click the button below. A native file picker opens.
2. Select a macOS package (e.g. any .app bundle from /Applications). FileSelectHelper will zip it into a temp file and add it to temporary_files_.
3. The page closes itself ~100 ms after the picker returns. This destroys the WebContents while the enterprise content-analysis scan is still in flight.