src: configure v8 isolate with uv_get_constrained_memory

This change adds the ability to set the memory ceiling for a Node.js process
according to a memory limit set by cgroups (via uv_get_constrained_memory),
which is used by docker containers to set resource constraints. Previously we
would use the physical memory size to estimate the necessary V8 heap sizes, but
the physical memory size is not necessarily the correct limit, e.g. if the
process is running inside a docker container or is otherwise constrained.

Non-Linux systems shouldn't be affected.

src/api/environment.cc

@@ -179,7 +179,10 @@ void FreeArrayBufferAllocator(ArrayBufferAllocator* allocator) {
 }
 
 void SetIsolateCreateParamsForNode(Isolate::CreateParams* params) {
-  const uint64_t total_memory = uv_get_total_memory();
+  const uint64_t constrained_memory = uv_get_constrained_memory();
+  const uint64_t total_memory = constrained_memory > 0 ?
+      std::min(uv_get_total_memory(), constrained_memory) :
+      uv_get_total_memory();
   if (total_memory > 0) {
     // V8 defaults to 700MB or 1.4GB on 32 and 64 bit platforms respectively.
     // This default is based on browser use-cases. Tell V8 to configure the