There are a few minor differences between this and Mozilla's implementation:
(a) The type error messages were not hardcoded to the Performance class and were moved to Errors.msg or domerr.msg instead.
(b) Resist fingerprinting-pref changes were skipped.
Partially based on https://bugzilla.mozilla.org/show_bug.cgi?id=1724645
This adds the later spec revision's contentBoxSize and borderBoxSize, and the
inlineSize and blockSize concepts.
The older spec contentRect remains functional as well for backwards
compatibility with the earlier spec (that was IMHO perfectly fine as it was...)
This was added as part of the WebExtension sec issue CVE-2017-5389.
Since we no longer have this interface, it needed to be removed to
prevent errors in the WebRequest channel callback.
