Print memory usage in dex2oat shutdown

Example on mako:
I/dex2oat (31071): dex2oat took 31.195s (threads: 2) arena alloc=1013KB java alloc=13MB native alloc=32MB free=1490KB

Bug: 18069309

Change-Id: I08eac00842be35d4e659bddc8513f2062be725c9

(cherry picked from commit 3029df6d212894647ba0e5c23443c40912c6ecc8)
diff --git a/compiler/utils/arena_allocator.cc b/compiler/utils/arena_allocator.cc
index 004af98..a80ad93 100644
--- a/compiler/utils/arena_allocator.cc
+++ b/compiler/utils/arena_allocator.cc
@@ -189,6 +189,15 @@
   return ret;
 }
 
+size_t ArenaPool::GetBytesAllocated() const {
+  size_t total = 0;
+  MutexLock lock(Thread::Current(), lock_);
+  for (Arena* arena = free_arenas_; arena != nullptr; arena = arena->next_) {
+    total += arena->GetBytesAllocated();
+  }
+  return total;
+}
+
 void ArenaPool::FreeArenaChain(Arena* first) {
   if (UNLIKELY(RUNNING_ON_VALGRIND > 0)) {
     for (Arena* arena = first; arena != nullptr; arena = arena->next_) {