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

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index e0e62fa0237d6581e586a4d7098810986311266a..22a4cedd50d5fde50fd2cccef881470e2a020d9c 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -305,6 +305,9 @@ class OS {
   // positions indicated by the members of the CpuFeature enum from globals.h
   static uint64_t CpuFeaturesImpliedByPlatform();
 
+  // The total amount of physical memory available on the current system.
+  static uintptr_t TotalPhysicalMemory();
+
   // Maximum size of the virtual memory.  0 means there is no artificial
   // limit.
   static intptr_t MaxVirtualMemory();