UnsafeArena not defined in external header files

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I have an application that instantiates many different TemplateDictionary 
objects. I would like to explicitly pass in an UnsafeArena object to the 
constructor so that I can share a single arena across dictionaries instead of 
allocating a 32KB arena for each object.

Unfortunately, the definition of UnsafeArena does not appear to be defined in 
the generated header files of ctemplate, so I can't instantiate an UnsafeArena 
object to pass it to the TemplateDictionary constructor.

Original issue reported on code.google.com by [email protected] on 11 Aug 2014 at 9:15