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

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index fe27eaf71f437fcc099ffdebb8e24f60c999921f..f4e11e733b1f3d12d233dfde2389a86530681b82 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -102,6 +102,48 @@ intptr_t OS::MaxVirtualMemory() {
 }
 
 
+uint64_t OS::TotalPhysicalMemory() {
+#if V8_OS_MACOSX
+  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) {
+    UNREACHABLE();
+    return 0;
+  }
+  return static_cast<uint64_t>(size);
+#elif V8_OS_FREEBSD
+  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<uint64_t>(pages) * page_size;
+#elif V8_OS_CYGWIN
+  MEMORYSTATUS memory_info;
+  memory_info.dwLength = sizeof(memory_info);
+  if (!GlobalMemoryStatus(&memory_info)) {
+    UNREACHABLE();
+    return 0;
+  }
+  return static_cast<uint64_t>(memory_info.dwTotalPhys);
+#else
+  intptr_t pages = sysconf(_SC_PHYS_PAGES);
+  intptr_t page_size = sysconf(_SC_PAGESIZE);
+  if (pages == -1 || page_size == -1) {
+    UNREACHABLE();
+    return 0;
+  }
+  return static_cast<uint64_t>(pages) * page_size;
+#endif
+}
+
+
 int OS::ActivationFrameAlignment() {
 #if V8_TARGET_ARCH_ARM
   // On EABI ARM targets this is required for fp correctness in the