This adds the --expose-experimental-interfaces command line flag to
enable experimental IDL interfaces. Any IDL interface with Experimental
in its exposed attributes will be disabled by default.
The problem is that by stubbing out or partially implementing interfaces
in LibWeb, we actually make some sites behave worse. For example, the
OffscreenCanvas interface being exposed makes sites believe we fully
support it, even though we don't. If the interface was not exposed,
these sites may fall back to ordinary canvas objects. Similarly, to
use YouTube, we currently have to patch out MSE interfaces.
This flag will allow developers to iteratively work on features,
without breaking such sites. We enable experimental interfaces during
tests.
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