/usr/share/vala-0.40/vapi/webkit2gtk-web-extension-4.0.deps is in valac-0.40-vapi 0.40.4-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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gtk+-3.0 javascriptcoregtk-4.0 libsoup-2.4