ART: Change InstructionSetFeatures to return unique_ptr

This makes clear the ownership of the object.

Test: m test-art-host
Change-Id: I55fa734f04bc3046d370f4dcf98ce6b17c59e234
diff --git a/compiler/common_compiler_test.cc b/compiler/common_compiler_test.cc
index 51bf9ea..2f9164c 100644
--- a/compiler/common_compiler_test.cc
+++ b/compiler/common_compiler_test.cc
@@ -156,7 +156,7 @@
 
     const InstructionSet instruction_set = kRuntimeISA;
     // Take the default set of instruction features from the build.
-    instruction_set_features_.reset(InstructionSetFeatures::FromCppDefines());
+    instruction_set_features_ = InstructionSetFeatures::FromCppDefines();
 
     runtime_->SetInstructionSet(instruction_set);
     for (int i = 0; i < Runtime::kLastCalleeSaveType; i++) {