Provide a mechanism for the embedder to set a TotalPhysicalMemory callback function.

src/defaults.cc

Index: src/defaults.cc
diff --git a/src/defaults.cc b/src/defaults.cc
index cbbe53729ebd171772106b589ae8b8c6fb4488d1..913d094ad59a2a6c97aed80ffa6f2f3a8959930b 100644
--- a/src/defaults.cc
+++ b/src/defaults.cc
@@ -50,7 +50,15 @@ bool ConfigureResourceConstraintsForCurrentPlatform(
     return false;
   }
 
-  uint64_t physical_memory = i::OS::TotalPhysicalMemory();
+  uint64_t physical_memory;
+  i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  if (isolate != NULL &&
+      isolate->thread_local_top()->total_physical_memory_callback_ != NULL) {
+    physical_memory =
+        isolate->thread_local_top()->total_physical_memory_callback_();
+  } else {
+    physical_memory = i::OS::TotalPhysicalMemory();
+  }
   int lump_of_memory = (i::kPointerSize / 4) * i::MB;
 
   // The young_space_size should be a power of 2 and old_generation_size should