[Isolates] Use TLS for global isolate storage by default.

src/isolate.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
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 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef V8_ISOLATE_H_
 #define V8_ISOLATE_H_
 
-// #define V8_USE_TLS_FOR_GLOBAL_ISOLATE
+#define V8_USE_TLS_FOR_GLOBAL_ISOLATE
 
 #include "apiutils.h"
 #include "contexts.h"
 #include "execution.h"
 #include "frames-inl.h"
 #include "frames.h"
 #include "handles.h"
 #include "heap.h"
 #include "zone.h"
 
@@ skipping 687 matching lines @@
 
 // Temporary macro to be used to flag classes that are completely converted
 // to be isolate-friendly. Their mix of static/nonstatic methods/fields is
 // correct.
 #define ISOLATED_CLASS class
 
 } }  // namespace v8::internal
 
 
 #endif  // V8_ISOLATE_H_