Start implementing jdb "locals".

This lets us show the names and types of the locals, but all the values
will show up as 0/null. We're going to have to walk the whole stack and
take callee-save frames into account to do that right.

Change-Id: Ic6e115513b6e65ae7ed4b7274e70bc514e83190a
diff --git a/src/check_jni.cc b/src/check_jni.cc
index b6670e4..659dcb8 100644
--- a/src/check_jni.cc
+++ b/src/check_jni.cc
@@ -818,7 +818,7 @@
       // TODO
       break;
     case kString:
-      okay = obj->IsString();
+      okay = obj->GetClass()->IsStringClass();
       break;
     case kThrowable:
       // TODO