Give the JIT its own arena pool to avoid lock contentions.

Sharing it with the verifier and the class loader is not ideal,
especially at startup time.

bug:27398183
bug:23128949
Change-Id: I1b91663a13f6c5b33ad3b4be780d93eb7fe445b4
diff --git a/compiler/jit/jit_compiler.cc b/compiler/jit/jit_compiler.cc
index 23601c3..79a6d38 100644
--- a/compiler/jit/jit_compiler.cc
+++ b/compiler/jit/jit_compiler.cc
@@ -230,10 +230,10 @@
   }
 
   // Trim maps to reduce memory usage.
-  // TODO: measure how much this increases compile time.
+  // TODO: move this to an idle phase.
   {
     TimingLogger::ScopedTiming t2("TrimMaps", &logger);
-    runtime->GetArenaPool()->TrimMaps();
+    runtime->GetJitArenaPool()->TrimMaps();
   }
 
   total_time_ += NanoTime() - start_time;