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/arm/assembler_arm.cc b/src/oat/utils/arm/assembler_arm.cc
index e248d5f..55b6187 100644
--- a/src/oat/utils/arm/assembler_arm.cc
+++ b/src/oat/utils/arm/assembler_arm.cc
@@ -1651,6 +1651,10 @@
   UNIMPLEMENTED(FATAL) << "no sign extension necessary for arm";
 }
 
+void ArmAssembler::ZeroExtend(ManagedRegister /*mreg*/, size_t /*size*/) {
+  UNIMPLEMENTED(FATAL) << "no zero extension necessary for arm";
+}
+
 void ArmAssembler::Move(ManagedRegister m_dst, ManagedRegister m_src, size_t /*size*/) {
   ArmManagedRegister dst = m_dst.AsArm();
   ArmManagedRegister src = m_src.AsArm();