libbacktrace: expose BacktraceMap's unwindstack::Memory.

Test: mma
Change-Id: Icd2b891b121b90d55e3ac45037a59c24221a2496
diff --git a/libbacktrace/UnwindStackMap.cpp b/libbacktrace/UnwindStackMap.cpp
index a9121a8..e7e5e4c 100644
--- a/libbacktrace/UnwindStackMap.cpp
+++ b/libbacktrace/UnwindStackMap.cpp
@@ -96,6 +96,10 @@
   return name;
 }
 
+std::shared_ptr<unwindstack::Memory> UnwindStackMap::GetProcessMemory() {
+  return process_memory_;
+}
+
 //-------------------------------------------------------------------------
 // BacktraceMap create function.
 //-------------------------------------------------------------------------