Make PartitionAlloc build on Gecko.
Move PartitionAlloc to js/src/ for testing on the JS engine.

Also remove a few unused headers, and tweak some compat macros.
Use PartitionAlloc for all SpiderMonkey allocations.
Disable PartitionAlloc when MOZ_MEMORY is not defined.
Replace use of ScopedFreePtr with UniquePtr in SpiderMonkey.

ScopedFreePtr is deprecated and uses |free| directly, as opposed to
UniquePtr, which can be made to use js_free.
Use js_calloc for allocating ArrayBufferContents objects.
Copy sandbox option strings instead of adopting them.

Strings coming from JS objects are allocated on a separate heap,
but nsCString is unaware of that, and tries to free its underlying
buffer using a regular free(); because of that, we need to copy the
string into a separate buffer allocated from the correct heap.
Use js_realloc in nsXMLHttpRequest::ArrayBufferBuilder.
Use js_free in ThreadSharedFloatArrayBufferList.

This may be wrong, as the underlying buffer seems to be entirely internal
and not exposed to content (in fact, it is immutable). Perhaps the proper
solution to this would be to copy the buffer in

StealJSArrayDataIntoThreadSharedFloatArrayBufferList

in AudioBuffer.cpp