Throw a more descriptive exception when blocking 'eval' via CSP.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index cfc576cecb6fedb6d83d893650824f39ed55cccf..28e4af53682b2fa6a55623914720d7939bbd0285 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -154,6 +154,8 @@ enum BindingFlags {
   V(MAP_CACHE_INDEX, Object, map_cache) \
   V(CONTEXT_DATA_INDEX, Object, data) \
   V(ALLOW_CODE_GEN_FROM_STRINGS_INDEX, Object, allow_code_gen_from_strings) \
+  V(ERROR_MESSAGE_FOR_CODE_GEN_FROM_STRINGS_INDEX, Object, \
+    error_message_for_code_gen_from_strings) \
   V(TO_COMPLETE_PROPERTY_DESCRIPTOR_INDEX, JSFunction, \
     to_complete_property_descriptor) \
   V(DERIVED_HAS_TRAP_INDEX, JSFunction, derived_has_trap) \
@@ -281,6 +283,7 @@ class Context: public FixedArray {
     OUT_OF_MEMORY_INDEX,
     CONTEXT_DATA_INDEX,
     ALLOW_CODE_GEN_FROM_STRINGS_INDEX,
+    ERROR_MESSAGE_FOR_CODE_GEN_FROM_STRINGS_INDEX,
     TO_COMPLETE_PROPERTY_DESCRIPTOR_INDEX,
     DERIVED_HAS_TRAP_INDEX,
     DERIVED_GET_TRAP_INDEX,
@@ -381,6 +384,8 @@ class Context: public FixedArray {
   Object* OptimizedFunctionsListHead();
   void ClearOptimizedFunctions();
 
+  Handle<Object> ErrorMessageForCodeGenerationFromStrings();
+
 #define NATIVE_CONTEXT_FIELD_ACCESSORS(index, type, name) \
   void  set_##name(type* value) {                         \
     ASSERT(IsNativeContext());                            \