Move Assemblers to the Arena.

And clean up some APIs to return std::unique_ptr<> instead
of raw pointers that don't communicate ownership.

Change-Id: I3017302307a0253d661240750298802fb0d9585e
diff --git a/compiler/utils/x86/assembler_x86_test.cc b/compiler/utils/x86/assembler_x86_test.cc
index d0d5147..1d1df6e 100644
--- a/compiler/utils/x86/assembler_x86_test.cc
+++ b/compiler/utils/x86/assembler_x86_test.cc
@@ -16,13 +16,16 @@
 
 #include "assembler_x86.h"
 
+#include "base/arena_allocator.h"
 #include "base/stl_util.h"
 #include "utils/assembler_test.h"
 
 namespace art {
 
 TEST(AssemblerX86, CreateBuffer) {
-  AssemblerBuffer buffer;
+  ArenaPool pool;
+  ArenaAllocator arena(&pool);
+  AssemblerBuffer buffer(&arena);
   AssemblerBuffer::EnsureCapacity ensured(&buffer);
   buffer.Emit<uint8_t>(0x42);
   ASSERT_EQ(static_cast<size_t>(1), buffer.Size());