Jni compiler should zero extend boolean and char return values.

This mostly fixes the emulator. It seems like there's some final issues
in browser.

Change-Id: I9a82db3c243a0994465106bf21ac6596af5bdf5a
diff --git a/src/oat/utils/assembler.h b/src/oat/utils/assembler.h
index 71f32c0..dabd321 100644
--- a/src/oat/utils/assembler.h
+++ b/src/oat/utils/assembler.h
@@ -401,6 +401,9 @@
   // Sign extension
   virtual void SignExtend(ManagedRegister mreg, size_t size) = 0;
 
+  // Zero extension
+  virtual void ZeroExtend(ManagedRegister mreg, size_t size) = 0;
+
   // Exploit fast access in managed code to Thread::Current()
   virtual void GetCurrentThread(ManagedRegister tr) = 0;
   virtual void GetCurrentThread(FrameOffset dest_offset,