ART: Change image_classes and compiled_classes to unordered set

These lists can be large, and the soon-to-follow compiled_methods
will be potentially even larger. Use a hash set instead of a tree
set.

Change-Id: I7d25c075540f47771354c6f49928b4fd0c76eb2e
diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index cfd525c..8016831 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -124,7 +124,7 @@
   }
 
   ASSERT_TRUE(compiler_driver_->GetImageClasses() != NULL);
-  std::set<std::string> image_classes(*compiler_driver_->GetImageClasses());
+  std::unordered_set<std::string> image_classes(*compiler_driver_->GetImageClasses());
 
   // Need to delete the compiler since it has worker threads which are attached to runtime.
   compiler_driver_.reset();