[heap] Reland "Use RAIL mode for initial heap sizing"

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index e5f1766a831f47a52197e459b4858518dc6089ca..d26d22888209149890330df95a61ff08b595fe44 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1177,6 +1177,10 @@ class Isolate {
 
   void SetRAILMode(RAILMode rail_mode);
 
+  RAILMode rail_mode() { return rail_mode_.Value(); }
+
+  double LoadStartTimeMs();
+
   void IsolateInForegroundNotification();
 
   void IsolateInBackgroundNotification();
@@ -1362,6 +1366,8 @@ class Isolate {
   AccessCompilerData* access_compiler_data_;
   base::RandomNumberGenerator* random_number_generator_;
   base::AtomicValue<RAILMode> rail_mode_;
+  base::Mutex rail_mutex_;
+  double load_start_time_ms_;
 
   // Whether the isolate has been created for snapshotting.
   bool serializer_enabled_;