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/mips/assembler_mips.h b/compiler/utils/mips/assembler_mips.h
index ffac4c4..ecb67bd 100644
--- a/compiler/utils/mips/assembler_mips.h
+++ b/compiler/utils/mips/assembler_mips.h
@@ -102,8 +102,10 @@
 
 class MipsAssembler FINAL : public Assembler {
  public:
-  explicit MipsAssembler(const MipsInstructionSetFeatures* instruction_set_features = nullptr)
-      : overwriting_(false),
+  explicit MipsAssembler(ArenaAllocator* arena,
+                         const MipsInstructionSetFeatures* instruction_set_features = nullptr)
+      : Assembler(arena),
+        overwriting_(false),
         overwrite_location_(0),
         last_position_adjustment_(0),
         last_old_position_(0),