Add a helper function for allocation of deoptimization history.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 23242)
+++ runtime/vm/compiler.cc	(working copy)
@@ -499,11 +499,7 @@
         }
 
         // If not yet present, allocate deoptimization history array.
-        Array& deopt_history = Array::Handle(function.deopt_history());
-        if (deopt_history.IsNull()) {
-          deopt_history = Array::New(FLAG_deoptimization_counter_threshold);
-          function.set_deopt_history(deopt_history);
-        }
+        function.EnsureDeoptHistory();
 
         for (intptr_t i = 0; i < guarded_fields.length(); i++) {
           const Field& field = *guarded_fields[i];