Move global V8::UseCrankshaft() into the Isolate.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 8eace1268481c1011607548a610c37f1a31625f6..b50ab0e56e8d831b60c0b7a89ad10cbf67923591 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1059,6 +1059,8 @@ class Isolate {
     thread_local_top_.top_lookup_result_ = top;
   }
 
+  bool use_crankshaft() const { return use_crankshaft_; }
+
   bool initialized_from_snapshot() { return initialized_from_snapshot_; }
 
   double time_millis_since_init() {
@@ -1300,6 +1302,9 @@ class Isolate {
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   CodeStubInterfaceDescriptor* code_stub_interface_descriptors_;
 
+  // True if we are using the Crankshaft optimizing compiler.
+  bool use_crankshaft_;
+
   // True if this isolate was initialized from a snapshot.
   bool initialized_from_snapshot_;