Allocate optimizing compiler thread only when necessary.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b7ea209b295e6526d97675fef48da983fb7d8341..cfea075777386596e4cbe6acb02dcfdbd3506c4f 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1099,7 +1099,7 @@ class Isolate {
 #endif  // DEBUG
 
   OptimizingCompilerThread* optimizing_compiler_thread() {
-    return &optimizing_compiler_thread_;
+    return optimizing_compiler_thread_;
   }
 
   // PreInits and returns a default isolate. Needed when a new thread tries
@@ -1369,7 +1369,7 @@ class Isolate {
 #endif
 
   DeferredHandles* deferred_handles_head_;
-  OptimizingCompilerThread optimizing_compiler_thread_;
+  OptimizingCompilerThread* optimizing_compiler_thread_;
   MarkingThread** marking_thread_;
   SweeperThread** sweeper_thread_;