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/arm/assembler_thumb2.cc b/compiler/utils/arm/assembler_thumb2.cc
index 15298b3..26f7d0d 100644
--- a/compiler/utils/arm/assembler_thumb2.cc
+++ b/compiler/utils/arm/assembler_thumb2.cc
@@ -59,8 +59,8 @@
     return;
   }
   // Create and fill in the fixup_dependents_.
-  assembler->fixup_dependents_.reset(new FixupId[number_of_dependents]);
-  FixupId* dependents = assembler->fixup_dependents_.get();
+  assembler->fixup_dependents_.resize(number_of_dependents);
+  FixupId* dependents = assembler->fixup_dependents_.data();
   for (FixupId fixup_id = 0u; fixup_id != end_id; ++fixup_id) {
     uint32_t target = fixups[fixup_id].target_;
     if (target > fixups[fixup_id].location_) {