AdjustExternalMemory

[jasongin]

While porting the canvas package to NAPI, I found the canvas code calls Nan::AdjustExternalMemory() in a few places, to give some hints to the V8 GC. That NAN API is a wrapper around v8::Isolate::AdjustAmountOfExternalAllocatedMemory(). It's not critical for the functionality, but I assume it improves GC behavior significantly when working with large amounts of externally-allocated memory (image buffers in the case of canvas).

This is a V8-specific API; JSRT does not appear to have an equivalent. Do we need to expose it in NAPI? Possibly it could be implemented as a no-op on other engines.

[mhdawson]

nodejs/node#13928

[digitalinfinity]

I think this has been addressed in nodejs/node#14310. Feel free to reopen nodejs/node#13928 if you disagree.