`webxr` depends on a very old verison of `time`, which allowed serializing
monotonic clock output. This isn't possible on all platforms, so newer
versions of `time` do not allow this. In order to stop using the old
0.1 versions of `time` we have to stop relying on times passed from `webxr`
to Servo. This change does that, at the cost of removing the XR
profiling feature. It has to be rewritten in another way in the `webxr`
crate.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>