SerfThreadFn might compile with the wrong calling convention and crash on shutdown

[GoogleCodeExporter]

As the serf url fetcher uses apr threads to create and join threads the thread 
function should have the correct signature:

http://code.google.com/p/modpagespeed/source/browse/branches/latest-stable/src/n
et/instaweb/apache/serf_url_async_fetcher.cc#789

static void* SerfThreadFn(apr_thread_t* thread_id, void* context)

Should be:
static APR_DECLARE(void*) SerfThreadFn(apr_thread_t* thread_id, void* context)

include apr.h for the APR_DECLARE

It would be better to use the ThreadSystem instance used in construction of the 
fetcher.

Original issue reported on code.google.com by kspoels...@we-amp.com on 30 Oct 2013 at 11:13