Give correct value ranges to enumeration types used as opaque types.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index d63ca5e617358f98510404b9abc9fd89af2bcdb4..81b7b3a6cd0e29c03d2b4088b1bac93439c0e78e 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -360,7 +360,13 @@ class ThreadHandle {
 class Thread: public ThreadHandle {
  public:
   // Opaque data type for thread-local storage keys.
-  enum LocalStorageKey {};
+  // LOCAL_STORAGE_KEY_MIN_VALUE and LOCAL_STORAGE_KEY_MAX_VALUE are specified
+  // to ensure that enumeration type has correct value range (see Issue 830 for
+  // more details).
+  enum LocalStorageKey {
+    LOCAL_STORAGE_KEY_MIN_VALUE = kMinInt,
+    LOCAL_STORAGE_KEY_MAX_VALUE = kMaxInt
+  };
 
   // Create new thread.
   Thread();