Fix some more shorten-64-to-32 warnings:

runtime/vm/timeline.h

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_TIMELINE_H_
 #define VM_TIMELINE_H_
 
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
 #include "vm/os.h"
@@ skipping 264 matching lines @@
   void set_pre_serialized_json(bool pre_serialized_json) {
     state_ = PreSerializedJSON::update(pre_serialized_json, state_);
   }
 
   enum StateBits {
     kEventTypeBit = 0,  // reserve 4 bits for type.
     kPreSerializedJSON = 4,
     kNextBit = 5,
   };
 
-  class EventTypeField : public BitField<EventType, kEventTypeBit, 4> {};
+  class EventTypeField : public BitField<uword, EventType, kEventTypeBit, 4> {};
   class PreSerializedJSON :
-      public BitField<bool, kPreSerializedJSON, 1> {};
+      public BitField<uword, bool, kPreSerializedJSON, 1> {};
 
   int64_t timestamp0_;
   int64_t timestamp1_;
   TimelineEventArgument* arguments_;
   intptr_t arguments_length_;
   uword state_;
   const char* label_;
   const char* category_;
   ThreadId thread_;
   Dart_Port isolate_id_;
@@ skipping 447 matching lines @@
 
  private:
   TimelineEventBlock* current_;
   TimelineEventRecorder* recorder_;
 };
 
 
 }  // namespace dart
 
 #endif  // VM_TIMELINE_H_