As part of the normalization process for WebExtension API calls, we need to
extract and validate the full set of value properties (including properties
X-rays would normally deny access to) from cross-compartment objects. This
currently involves waiving X-rays, enumerating property descriptors, and
unwaiving X-rays - all through X-ray wrappers and waivers - and generating a
lot of expensive and short-lived wrappers in-between.
This helper reads out the list of safe properties from within the object's
compartment, and then copies them over to an object in the target compartment,
without any X-ray overhead, or any unnecessary intermediate wrappers or
compartment switches. It cuts about 40% off the overhead of our normalization
code.
MozReview-Commit-ID: H582oAYocaX
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extra : rebase_source : 7f7d5df605bc6544cb7f1c0c7e224d81b211e09c
extra : histedit_source : f980a03413b5e65fc6fa272c012a769d2764d89b
13e61f79f9