Auto merge of #307 - servo:36247, r=jdm

Wrap uses of extern statics in unsafe blocks.

This fixes safe_extern_statics warnings; see
<rust-lang/rust#36247>.

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Author: bors-servo

src/rust.rs

@@ -440,11 +440,15 @@ impl<T> DerefMut for MutableHandle<T> {
 
 impl HandleValue {
     pub fn null() -> HandleValue {
-        NullHandleValue
+        unsafe {
+            NullHandleValue
+        }
     }
 
     pub fn undefined() -> HandleValue {
-        UndefinedHandleValue
+        unsafe {
+            UndefinedHandleValue
+        }
     }
 }
 
@@ -459,7 +463,11 @@ impl HandleObject {
 }
 
 impl Default for jsid {
-    fn default() -> jsid { JSID_VOID }
+    fn default() -> jsid {
+        unsafe {
+            JSID_VOID
+        }
+    }
 }
 
 impl Default for Value {