Add methods to enable configuration of ResourceConstraints based on limits derived at runtime.

src/platform-macos.cc

Index: src/platform-macos.cc
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index a58bc1aca3943184d0185246c5498a060b90149b..3b273b6120a585eb1633ee110e608e745154c405 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -202,6 +202,19 @@ void OS::SignalCodeMovingGC() {
 }
 
 
+uintptr_t OS::TotalPhysicalMemory() {
+  int mib[2];
+  mib[0] = CTL_HW;
+  mib[1] = HW_MEMSIZE;
+  int64_t size = 0;
+  size_t len = sizeof(size);
+  if (sysctl(mib, 2, &size, &len, NULL, 0) == 0)
+    return static_cast<uintptr_t> size;
+  UNREACHABLE();
+  return 0;
+}
+
+
 const char* OS::LocalTimezone(double time) {
   if (std::isnan(time)) return "";
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));