The standard placement new function is declared to not throw, which
means that, per spec, a null check on its result is required. There are
a number of places throughout xpcom/ where we know that we are passing
non-null pointers to placement new (and receiving them as a return
value), and we are therefore doing useless work performing these null
checks.
Therefore, we should be using an operator new overload that doesn't
require the null check. MFBT has just such an overload, so use that.
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