Extensions to check JNI.

Ensure critical lock isn't held when returning from a down-call.
Log a warning if the critical lock is held for a significant period of
time.
Refactor JNIEnvExt to be a class rather than a struct.

Test: mma test-art-host

Change-Id: I4d149cb04d3a7308a22b92b196e51e2f1ae17ede
diff --git a/runtime/scoped_thread_state_change.h b/runtime/scoped_thread_state_change.h
index 02b6124..0c42c5a 100644
--- a/runtime/scoped_thread_state_change.h
+++ b/runtime/scoped_thread_state_change.h
@@ -27,7 +27,7 @@
 namespace art {
 
 class JavaVMExt;
-struct JNIEnvExt;
+class JNIEnvExt;
 template<class MirrorType> class ObjPtr;
 class Thread;