Refactor to share map_info amongst threads.

Allow the use of the same map info to be shared when getting information on
multiple threads from the same pid.

Change-Id: I2e460e20154a10f4894ae563331fb32179e4551f
diff --git a/libbacktrace/BacktraceThread.h b/libbacktrace/BacktraceThread.h
index 8ed1122..bcb56c9 100644
--- a/libbacktrace/BacktraceThread.h
+++ b/libbacktrace/BacktraceThread.h
@@ -71,7 +71,8 @@
   // the compiler to catch if an implementation does not properly
   // subclass both.
   BacktraceThread(
-      BacktraceImpl* impl, BacktraceThreadInterface* thread_intf, pid_t tid);
+      BacktraceImpl* impl, BacktraceThreadInterface* thread_intf, pid_t tid,
+      backtrace_map_info_t* map_info);
   virtual ~BacktraceThread();
 
   virtual bool Unwind(size_t num_ignore_frames);