[SPARK-3957]: show broadcast variable resource usage info in UI

show the broadcast variables resource usage in UI

![image](https://cloud.githubusercontent.com/assets/678008/4696076/6bb23818-57e5-11e4-83b9-a59ced0d1bc4.png)

Is there a reason to make this serializable ? There are subclasses which are explicitly marked as non serializable https://github.com/CodingCat/spark/blob/SPARK-3957/core/src/main/scala/org/apache/spark/storage/BlockId.scala#L89 @aarondav can probably comment ?