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

src/platform-freebsd.cc

Index: src/platform-freebsd.cc
diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index 518ad3185f01c026fcf358261b7ad3fb964b871f..9d86f1349c70af3f3356df76d4071ea91940e2e7 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -199,6 +199,18 @@ void OS::SignalCodeMovingGC() {
 }
 
 
+uintptr_t OS::TotalPhysicalMemory() {
+  int pages, page_size;
+  size_t size = sizeof(pages);
+  sysctlbyname("vm.stats.vm.v_page_count", &pages, &size, NULL, 0);
+  sysctlbyname("vm.stats.vm.v_page_size", &page_size, &size, NULL, 0);
+  if (pages == -1 || page_size == -1) {
+    UNREACHABLE();
+    return 0;
+  }
+  return static_cast<uintptr_t>(pages) * page_size;
+}
+
 
 // Constants used for mmap.
 static const int kMmapFd = -1;